Fix subscription.open get called twice in the client libraries [(#1321)](GoogleCloudPlatform/python-docs-samples#1321)

Author: luxiangu

samples/snippets/subscriber.py

@@ -124,9 +124,8 @@ def callback(message):
 
     # Blocks the thread while messages are coming in through the stream. Any
     # exceptions that crop up on the thread will be set on the future.
-    future = subscription.open(callback)
     try:
-        future.result()
+        subscription.future.result()
     except Exception as e:
         print(
             'Listening for messages on {} threw an Exception: {}.'.format(