[1.7.x] Added missing request parameter to get_current_site().

Backport of d6c9bdd from master

docs/ref/contrib/sites.txt

@@ -486,7 +486,7 @@ the ``domain`` and ``name`` by looking at the request's domain. It has
 Finally, to avoid repetitive fallback code, the framework provides a
 :func:`django.contrib.sites.shortcuts.get_current_site` function.
 
-.. function:: shortcuts.get_current_site
+.. function:: shortcuts.get_current_site(request)
 
     A function that checks if ``django.contrib.sites`` is installed and
     returns either the current :class:`~django.contrib.sites.models.Site`