Updated example of customized ModelAdmin in documentation for 1.4

The change_view method of django.contrib.admin.ModelAdmin takes an
extra `form_url` argument in Django 1.4.

docs/ref/contrib/admin/index.txt

@@ -1266,11 +1266,11 @@ provided some extra mapping data that would not otherwise be available::
             # ...
             pass
 
-        def change_view(self, request, object_id, extra_context=None):
+        def change_view(self, request, object_id, form_url='', extra_context=None):
             extra_context = extra_context or {}
             extra_context['osm_data'] = self.get_osm_info()
             return super(MyModelAdmin, self).change_view(request, object_id,
-                extra_context=extra_context)
+                form_url, extra_context=extra_context)
 
 .. versionadded:: 1.4