doc: add missing comma

Adds comma after natural pause in sentence.

PR-URL: #16204
Reviewed-By: Joyee Cheung <[email protected]>
Reviewed-By: Gireesh Punathil <[email protected]>
Reviewed-By: Luigi Pinca <[email protected]>
Reviewed-By: Daijiro Wachi <[email protected]>
Reviewed-By: Colin Ihrig <[email protected]>
Reviewed-By: Anna Henningsen <[email protected]>
Reviewed-By: Yuta Hiroto <[email protected]>

Author: maclover7

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