doc: use HTTPS URL for suggested upstream remote

It's impossible to push to git:// URLs.

PR-URL: https://github.com/nodejs/node/pull/13602
Fixes: https://github.com/nodejs/node/issues/13600


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Brian White <mscdex@mscdex.net>