DOC: move mention of #21115 from bugs to enhancements (#21165)

Author: toobaz

doc/source/whatsnew/v0.23.1.txt

@@ -15,6 +15,8 @@ and bug fixes. We recommend that all users upgrade to this version.
 New features
 ~~~~~~~~~~~~
 
+- :meth:`Index.droplevel` is now implemented also for flat indexes, for compatibility with MultiIndex (:issue:`21115`)
+
 
 .. _whatsnew_0231.deprecations:
 
@@ -75,7 +77,6 @@ Indexing
 ^^^^^^^^
 
 - Bug in :meth:`Series.reset_index` where appropriate error was not raised with an invalid level name (:issue:`20925`)
-- :meth:`Index.droplevel` is now implemented also for flat indexes, for compatibility with MultiIndex (:issue:`21115`)
 -
 
 I/O