doc: fix minor typo in cluster.md

Adds a missing `'` in code example.

PR-URL: https://github.com/nodejs/node/pull/14353
Fixes: https://github.com/nodejs/node/issues/14352
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>

doc/api/cluster.md

@@ -191,7 +191,7 @@ added: v0.7.0
 
 Similar to the `cluster.on('message')` event, but specific to this worker.
 
-Within a worker, `process.on('message)` may also be used.
+Within a worker, `process.on('message')` may also be used.
 
 See [`process` event: `'message'`][].