fix default heartbeat in example conf file to 60

listed as 600, not the true default of 60

(cherry picked from commit 19337af)

Author: notque

docs/rabbitmq.conf.example

@@ -213,7 +213,7 @@
 ## * http://rabbitmq.com/heartbeats.html
 ## * http://rabbitmq.com/networking.html
 ##
-# heartbeat = 600
+# heartbeat = 60
 
 ## Set the max permissible size of an AMQP frame (in bytes).
 ##