[3.0.x] Updated iTunes podcast format link in syndication docs.

Backport of 2014319 from master

docs/ref/contrib/syndication.txt

@@ -1033,7 +1033,7 @@ attributes. Thus, you can subclass the appropriate feed generator class
 (``Atom1Feed`` or ``Rss201rev2Feed``) and extend these callbacks. They are:
 
 .. _georss: http://georss.org/
-.. _itunes podcast format: https://www.apple.com/itunes/podcasts/specs.html
+.. _itunes podcast format: https://help.apple.com/itc/podcasts_connect/#/itcb54353390
 
 ``SyndicationFeed.root_attributes(self)``
     Return a ``dict`` of attributes to add to the root feed element