doc: use "pull request" instead of "PR" in packages.md

PR-URL: https://github.com/nodejs/node/pull/39213


Reviewed-By: Harshitha K P <harshitha014@gmail.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>