doc: var -> let / const in events.md

PR-URL: https://github.com/nodejs/node/pull/11810


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>