doc: add a example code to API doc property

example code added to the process.setgroups() API
doc property in process.md

PR-URL: https://github.com/nodejs/node/pull/35738


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>