doc: change "GitHub Repo" to "Code repository"

"GitHub Repo & Issue Tracker" changed to "Code repository and issue
tracker".

PR-URL: https://github.com/nodejs/node/pull/33985


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Matheus Marchini <mat@mmarchini.me>
Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Myles Borins <myles.borins@gmail.com>