doc: add emitter.off() to events.md

PR-URL: https://github.com/nodejs/node/pull/20291
Refs: https://github.com/nodejs/node/pull/17156


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>