DOC: minor correction to v0.23.2.txt (pandas-dev#21644)

Author: topper-123

doc/source/whatsnew/v0.23.2.txt

@@ -61,8 +61,8 @@ Fixed Regressions
 Performance Improvements
 ~~~~~~~~~~~~~~~~~~~~~~~~
 
-- Improved performance of membership checks in :class:`CategoricalIndex`
-  (i.e. ``x in ci``-style checks are much faster). :meth:`CategoricalIndex.contains`
+- Improved performance of membership checks in :class:`Categorical` and :class:`CategoricalIndex`
+  (i.e. ``x in cat``-style checks are much faster). :meth:`CategoricalIndex.contains`
   is likewise much faster (:issue:`21369`, :issue:`21508`)
 - Improved performance of :meth:`HDFStore.groups` (and dependent functions like
   :meth:`~HDFStore.keys`.  (i.e. ``x in store`` checks are much faster)