[libcxx] Fix a typo in documentation (#148557)

Author: kazutakahirata

libcxx/docs/ReleaseNotes/21.rst

@@ -85,7 +85,7 @@ Improvements and New Features
 - The ``flat_map::insert`` and ``flat_set::insert_range`` have been optimized, resulting in a performance improvement of up
   to 10x for inserting elements into a ``flat_map`` when the input range is a ``flat_map`` or a ``zip_view``.
 
-- ``ctype::tolower`` and ``ctype::toupper`` have been optimized, resulting in a 2x performane improvement.
+- ``ctype::tolower`` and ``ctype::toupper`` have been optimized, resulting in a 2x performance improvement.
 
 Deprecations and Removals
 -------------------------