doc: add instructions to update local git config

this code change updates pull-requests.md to instruct developers to
update their local git config instead of global. This way their global
git config won't be overwritten if they use different email ID for work
or personal or non-Github projects

PR-URL: https://github.com/nodejs/node/pull/19777


Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>