Updates documentation for stopping channel subscriptions (#598)

googleapiclient/channel.py

diff --git a/googleapiclient/channel.py b/googleapiclient/channel.py
index 8153fe7..3caee13 100644
--- a/googleapiclient/channel.py
+++ b/googleapiclient/channel.py
@@ -67,7 +67,7 @@
 
 Example of unsubscribing.
 
-  service.channels().stop(channel.body())
+  service.channels().stop(channel.body()).execute()
 """
 from __future__ import absolute_import