doc: correct args for cluster message event

This commit corrects the cluster message event signature.

Fixes: https://github.com/nodejs/node/issues/5764
PR-URL: https://github.com/nodejs/node/pull/7297
Reviewed-By: Brian White <mscdex@mscdex.net>

doc/api/cluster.markdown

@@ -485,8 +485,8 @@ The `addressType` is one of:
 
 ## Event: 'message'
 
-* `worker` {cluster.Worker}
 * `message` {Object}
+* `handle` {undefined|Object}
 
 Emitted when any worker receives a message.