Update Doc/whatsnew/3.15.rst

Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>

Author: rhettinger

Doc/whatsnew/3.15.rst

@@ -288,7 +288,7 @@ collections
 
 * Added :meth:`!collections.Counter.__xor__` and
   :meth:`!collections.Counter.__ixor__` to compute the symmetric difference
-  between Counter objects.
+  between :class:`~collections.Counter` objects.
   (Contributed by Raymond Hettinger in :gh:`138682`.)