doc: add missing comma

Adds comma after natural pause in sentence.

PR-URL: nodejs/node#16204
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Daijiro Wachi <daijiro.wachi@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>

doc/api/cluster.md

@@ -5,7 +5,7 @@
 > Stability: 2 - Stable
 
 A single instance of Node.js runs in a single thread. To take advantage of
-multi-core systems the user will sometimes want to launch a cluster of Node.js
+multi-core systems, the user will sometimes want to launch a cluster of Node.js
 processes to handle the load.
 
 The cluster module allows easy creation of child processes that all share