[1.7.x] Fixed #22457 -- Fixed contributing guide w/o github set-up

The published commnand was not working when the user did not have
the git client set-up with the public-key. Changed the contributing
guide to clone it from https instead.

Backport of 28102991 from master.

docs/internals/contributing/writing-code/unit-tests.txt

@@ -30,7 +30,7 @@ sample settings module that uses the SQLite database. To run the tests:
 
 .. code-block:: bash
 
-   $ git clone git@github.com:django/django.git django-repo
+   $ git clone https://github.com:django/django.git django-repo
    $ cd django-repo/tests
    $ PYTHONPATH=..:$PYTHONPATH ./runtests.py