[Pull Requests] Fix Symfony upstream URL as git:// isn't supported by Github anymore.

See: https://github.blog/2021-09-01-improving-git-protocol-security-github/

Author: alexandre-daubois

contributing/code/pull_requests.rst

@@ -99,7 +99,7 @@ Get the Symfony source code:
 .. code-block:: terminal
 
       $ cd symfony
-      $ git remote add upstream git://github.com/symfony/symfony.git
+      $ git remote add upstream https://github.com/symfony/symfony.git
 
 Check that the current Tests Pass
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