Fix @contextmanager example

[decorator-factory]

A @contextmanager with yield that's not in a try block is wrong 99% of the time.

See: https://docs.python.org/3/library/contextlib.html#contextlib.contextmanager
If the with block exits with an exception, the generator receives that exception at the yield point.

Also see: https://docs.astral.sh/ruff/rules/fallible-context-manager/