Update API-design-guidelines.md

Update the "sink" property description
Fix camaraproject#289

documentation/API-design-guidelines.md

@@ -1677,7 +1677,7 @@ The following table provides `/subscriptions` attributes
 | name           | type               | attribute description                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    | cardinality                  |
 |----------------|--------------------|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|------------------------------| 
 | protocol       | string             | Identifier of a delivery protocol. **Only** `HTTP` **is allowed for now**.                                                                                                                                                                                                                                                                                                                                                                                                                                                               | Mandatory                    |
-| sink           | string             | https callback address where the notification must be POST-ed                                                                                                                                                                                                                                                                                                                                                                                                                                                                            | mandatory                    |
+| sink           | string             | The address to which events shall be delivered, using the HTTP protocol.                                                                                                                                                                                                                                                                                                                                                                                                                                                                            | mandatory                    |
 | sinkCredential | object             | Sink credential provides authorization information necessary to enable delivery of events to a target. In order to be updated in future this object is polymorphic. See detail below. To protect the notification endpoint providing sinkCredential is RECOMMENDED.                                                                                                                                                                                                                                                                      | optional                     |
 | types          | string             | Type of event subscribed. This attribute **must** be present in the `POST` request. It is required by API project to provide an enum for this attribute. `type` must follow the format: `org.camaraproject.<api-name>.<api-version>.<event-name>` with the `api-version` with letter `v` and the major version like  ``org.camaraproject.device-roaming-subscriptions.v1.roaming-status`` - Note: An array of types could be passed **but as of now only one value MUST passed**. Use of multiple value will be open later at API level. | mandatory                    |
 | config         | object             | Implementation-specific configuration parameters needed by the subscription manager for acquiring events. In CAMARA we have predefined attributes like ``subscriptionExpireTime``, ``subscriptionMaxEvents`` or ``initialEvent``. See detail below.                                                                                                                                                                                                                                                                                      | mandatory                    |