doc: use HTTPS URL for suggested upstream remote

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

PR-URL: #13602
Fixes: #13600
Reviewed-By: Anna Henningsen <[email protected]>
Reviewed-By: Refael Ackermann <[email protected]>
Reviewed-By: Gibson Fahnestock <[email protected]>
Reviewed-By: Brian White <[email protected]>

Author: seishun

CONTRIBUTING.md

@@ -30,7 +30,7 @@ locally.
 ```text
 $ git clone [email protected]:username/node.git
 $ cd node
-$ git remote add upstream git://github.com/nodejs/node.git
+$ git remote add upstream https://github.com/nodejs/node.git
 ```
 
 #### Which branch?