doc: remove extra comma in cluster docs

PR-URL: https://github.com/nodejs/node/pull/8557
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Prince John Wesley <princejohnwesley@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

doc/api/cluster.md

@@ -43,7 +43,7 @@ $ NODE_DEBUG=cluster node server.js
 23521,Master Worker 23528 online
 ```
 
-Please note that, on Windows, it is not yet possible to set up a named pipe
+Please note that on Windows, it is not yet possible to set up a named pipe
 server in a worker.
 
 ## How It Works