[3.12] Fix definition of a `generator iterator` in `glossary.rst` (GH-128952) (#128968)

Fix definition of a `generator iterator` in `glossary.rst` (GH-128952)

Fix possible typo/grammar in glossary.rst

As discussed in https://discuss.python.org/t/typo-in-glossary-entry-for-generator-iterator/77163
(cherry picked from commit 8174770d311ba09c07a47cc3ae90a1db2e7d7708)

Co-authored-by: Daniel F Moisset <dfmoisset@gmail.com>