namespace prefixes: add link to w3.org

Author: ukarroum

Doc/whatsnew/3.15.rst

@@ -660,7 +660,7 @@ xml.parsers.expat
 
   .. _billion laughs: https://en.wikipedia.org/wiki/Billion_laughs_attack
 
-* Add support for namespace prefixes.
+* Add support for `namespace prefixes <https://www.w3.org/TR/xml-names/#dt-prefix>`_.
   (Contributed by Yassir Karroum in :gh:`118317`.)