doc: fix link in cluster documentation

PR-URL: https://github.com/nodejs/node/pull/5068
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/cluster.markdown

@@ -50,7 +50,7 @@ server in a worker.
 
 <!--type=misc-->
 
-The worker processes are spawned using the [`child_process.fork`][] method,
+The worker processes are spawned using the [`child_process.fork()`][] method,
 so that they can communicate with the parent via IPC and pass server
 handles back and forth.