lib: add comment to script eval _tickCallback

Add a comment to match lib/module.js, missed in #11958.

PR-URL: https://github.com/nodejs/node/pull/12050
Ref: https://github.com/nodejs/node/pull/11958


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>