doc: use serial comma in events docs

Refs: nodejs/node#11321
Refs: nodejs/node#17384

PR-URL: nodejs/node#43113
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Darshan Sen <raisinten@gmail.com>
Reviewed-By: LiviaMedeiros <livia@cirno.name>

doc/api/events.md

@@ -810,7 +810,7 @@ The [`--trace-warnings`][] command-line flag can be used to display the
 stack trace for such warnings.
 
 The emitted warning can be inspected with [`process.on('warning')`][] and will
-have the additional `emitter`, `type` and `count` properties, referring to
+have the additional `emitter`, `type`, and `count` properties, referring to
 the event emitter instance, the event’s name and the number of attached
 listeners, respectively.
 Its `name` property is set to `'MaxListenersExceededWarning'`.