Fixed a dead link in django/contrib/sitemaps/__init__.py.

django/contrib/sitemaps/__init__.py

@@ -46,7 +46,7 @@ def ping_google(sitemap_url=None, ping_url=PING_URL):
 
 class Sitemap(object):
     # This limit is defined by Google. See the index documentation at
-    # http://sitemaps.org/protocol.php#index.
+    # http://www.sitemaps.org/protocol.html#index.
     limit = 50000
 
     # If protocol is None, the URLs in the sitemap will use the protocol