doc: replace anonymous function with arrow function

PR-URL: https://github.com/nodejs/node/pull/24617


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Yuta Hiroto <hello@hiroppy.me>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>