timer: remove duplicated word in comment

PR-URL: https://github.com/nodejs/node/pull/11323


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>