Update subscriber example in README to current patterns. (#6194)

Closes #6189.

pubsub/README.rst

@@ -90,9 +90,9 @@ messages to it
 .. code-block:: python
 
     import os
-    from google.cloud import pubsub
+    from google.cloud import pubsub_v1
 
-    publisher = pubsub.PublisherClient()
+    publisher = pubsub_v1.PublisherClient()
     topic_name = 'projects/{project_id}/topics/{topic}'.format(
         project_id=os.getenv('GOOGLE_CLOUD_PROJECT'),
         topic='MY_TOPIC_NAME',  # Set this to something appropriate.
@@ -109,14 +109,14 @@ Subscribing
 ^^^^^^^^^^^
 
 To subscribe to data in Cloud Pub/Sub, you create a subscription based on
-the topic, and subscribe to that.
+the topic, and subscribe to that, passing a callback function.
 
 .. code-block:: python
 
     import os
-    from google.cloud import pubsub
+    from google.cloud import pubsub_v1
 
-    subscriber = pubsub.SubscriberClient()
+    subscriber = pubsub_v1.SubscriberClient()
     topic_name = 'projects/{project_id}/topics/{topic}'.format(
         project_id=os.getenv('GOOGLE_CLOUD_PROJECT'),
         topic='MY_TOPIC_NAME',  # Set this to something appropriate.
@@ -127,17 +127,22 @@ the topic, and subscribe to that.
     )
     subscriber.create_subscription(
         name=subscription_name, topic=topic_name)
-    subscription = subscriber.subscribe(subscription_name)
-
-The subscription is opened asychronously, and messages are processed by
-use of a callback.
-
-.. code-block:: python
 
     def callback(message):
         print(message.data)
         message.ack()
-    subscription.open(callback)
+
+    future = subscriber.subscribe(subscription_name, callback)
+
+The future returned by the call to ``subscriber.subscribe`` can be used to
+block the current thread until a given condition obtains:
+
+.. code-block:: python
+
+    try:
+        future.result()
+    except KeyboardInterrupt:
+        future.cancel()
 
 To learn more, consult the `subscriber documentation`_.