doc: simplify text in pull-requests.md

Mostly, this replaces "It is recommended to do X" with "Do X."

PR-URL: https://github.com/nodejs/node/pull/30458


Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Denys Otrishko <shishugi@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>