Fix argpraser for pubsub subscriber

Change-Id: I776863091846ee8ff8a70078c8b8d5498cf81ed6

Author: Jon Wayne Parrott

samples/snippets/subscriber.py

@@ -142,7 +142,7 @@ def callback(message):
         delete_subscription(
             args.project, args.subscription_name)
     elif args.command == 'receive':
-        receive_messages(args.project, args.topic_name, args.subscription_name)
+        receive_messages(args.project, args.subscription_name)
     elif args.command == 'receive-flow-control':
         receive_messages_with_flow_control(
-            args.project, args.topic_name, args.subscription_name)
+            args.project, args.subscription_name)