Don't include port in Pub/Sub listener docs region

The 8080 port is only relevant for deploying this sample application
to Google App Engine flexible environment.

Author: remi Taylor

pubsub/listener.rb

@@ -23,6 +23,6 @@
   logger.info "Pushed Message: #{data}"
   response.status = 204
 end
+# [END receive_push_message]
 
 set :port, 8080
-# [END receive_push_message]