doc: add missing property in cluster example

`Cluster.setupMaster(options)` Options object was missing an `args`
property on the example.

PR-URL: https://github.com/nodejs/node/pull/5305
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>

doc/api/cluster.markdown

@@ -624,6 +624,7 @@ cluster.setupMaster({
 });
 cluster.fork(); // https worker
 cluster.setupMaster({
+  exec: 'worker.js',
   args: ['--use', 'http']
 });
 cluster.fork(); // http worker