Fix generic default documentation by George-Ogden · Pull Request #2265 · python/typing

Change the version defaults were added from Python 3.12 to Python 3.13

Pep 0695: https://peps.python.org/pep-0695/ says that the Python version is 3.12; however, it says in the docs that this was added in Python 3.13 (at the bottom of this section https://docs.python.org/3/library/typing.html#typing.TypeVar), and I'm getting a syntax error when I test this in Python 3.12.