doc: fix link in cluster documentation

PR-URL: #5068
Reviewed-By: Colin Ihrig <[email protected]>
Reviewed-By: Brian White <[email protected]>
Reviewed-By: James M Snell <[email protected]>

Author: TimothyGu

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.