Fixed #17749 - Documented better way of overriding ModelAdmin; thanks chrisdpratt and claudep.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@17582 bcc190cf-cafb-0310-a4f2-bffc1f526a37

docs/ref/contrib/admin/index.txt

@@ -1278,11 +1278,10 @@ provided some extra mapping data that would not otherwise be available::
             pass
 
         def change_view(self, request, object_id, extra_context=None):
-            my_context = {
-                'osm_data': self.get_osm_info(),
-            }
+            extra_context = extra_context or {}
+            extra_context['osm_data'] = self.get_osm_info()
             return super(MyModelAdmin, self).change_view(request, object_id,
-                extra_context=my_context)
+                extra_context=extra_context)
 
 .. versionadded:: 1.4