Correct Glossary key function example to use str.casefold instead of str.lower

The glossary currently states:

“the str.lower() method can serve as a key function for case-insensitive sorts.”

I believe this should be updated to use str.casefold() instead.
str.casefold() is recommended for case-insensitive matching in Unicode, and it provides more accurate behavior across languages than str.lower(). This also aligns with the "Sorting Techniques" section of the docs, which uses str.casefold() as the key function for case-insensitive sorting.

Author: cbkinase

Doc/glossary.rst

@@ -813,7 +813,7 @@ 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