Replace the term Immutable with a Hashable in the sequence entry of the Glossary (#124350)

The term `Immutable` in the `sequence` entry of the glossary is used incorrectly, in fact dicts accepts hashable keys, which is not the same as immutable.

Author: decorator-factory

Doc/glossary.rst

@@ -1131,7 +1131,7 @@ Glossary
       :class:`tuple`, and :class:`bytes`. Note that :class:`dict` also
       supports :meth:`~object.__getitem__` and :meth:`!__len__`, but is considered a
       mapping rather than a sequence because the lookups use arbitrary
-      :term:`immutable` keys rather than integers.
+      :term:`hashable` keys rather than integers.
 
       The :class:`collections.abc.Sequence` abstract base class
       defines a much richer interface that goes beyond just