doc: fix typos in CONTRIBUTING.md

Remove incorrect usage of "in which". The sentence is better and shorter
without it anyway. Replace incorrect "with" with "in".

PR-URL: https://github.com/nodejs/node/pull/17930


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Weijia Wang <starkwang@126.com>
Reviewed-By: Jon Moss <me@jonathanmoss.me>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>