doc: add missing property in cluster example

MrOutput · rvagg · commit f8c6701e22fe · 2016-02-28T12:41:47.000+11:00
`Cluster.setupMaster(options)` Options object was missing an `args` property on the example. PR-URL: #5305 Reviewed-By: James M Snell <jasnell@gmail.com> Reviewed-By: Roman Reiss <me@silverwind.io>
diff --git a/doc/api/cluster.markdown b/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
