Correct formatting in whatsnew entry

Author: MarcoGorelli

doc/source/whatsnew/v0.25.1.rst

@@ -21,7 +21,7 @@ Other enhancements
 Bug fixes
 ~~~~~~~~~
 
-- Bug in :func:`fill_na` in :class:`Categorical` would replace all values, not just those that are NaN (:issue:`26215`)
+- Bug in :func:`fill_na` in :meth:Categorical.fillna would replace all values, not just those that are NaN (:issue:`26215`)
 
 
 Categorical