/invitation-accepted Reword description

This patch makes it clearer what the meaning of the status code is

spec.yaml

@@ -233,7 +233,7 @@ paths:
           schema:
             $ref: "#/definitions/Error"
         409:
-          description: User is already known by the OCM provider.
+          description: Invitation already accepted.
           schema:
             $ref: "#/definitions/Error"