doc: use arrow function for anonymous callbacks

PR-URL: https://github.com/nodejs/node/pull/24606


Reviewed-By: Ron Korving <ron@ronkorving.nl>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>