[3.13] Correct Glossary key function example to use str.casefold instead of str.lower (GH-140316) (#142806)

Co-authored-by: Cameron Beck <94604154+cbkinase@users.noreply.github.com>

Author: miss-islington

Doc/glossary.rst

@@ -767,7 +767,7 @@ Glossary
       :func:`itertools.groupby`.
 
       There are several ways to create a key function.  For example. the
-      :meth:`str.lower` method can serve as a key function for case insensitive
+      :meth:`str.casefold` method can serve as a key function for case insensitive
       sorts.  Alternatively, a key function can be built from a
       :keyword:`lambda` expression such as ``lambda r: (r[0], r[2])``.  Also,
       :func:`operator.attrgetter`, :func:`operator.itemgetter`, and