Fixed issue reporting link in 404 template

This change directs users to report website issues on GitHub rather than
on Trac. Trac is for issues about the Django web framework, not the
Django website.

Author: adamzap

djangoproject/templates/404.html

@@ -11,7 +11,7 @@ <h2>{% translate "Page not found" %}</h2>
   <p>
     {% blocktranslate trimmed %}
       Looks like you followed a bad link. If you think it's our fault, please
-      <a href="https://code.djangoproject.com/">let us know</a>.
+      <a href="https://github.com/django/djangoproject.com/issues/">let us know</a>.
     {% endblocktranslate %}</p>
 
   {% url 'homepage' as homepage_url %}