[3.14] gh-145870: Fix Format.SOURCE reference in get_annotations docstring (GH-145889) (GH-146036)

The get_annotations() docstring incorrectly referred to the SOURCE
format, which was renamed to STRING during PEP 749 development.
(cherry picked from commit 2a0fa500f82fc160feb726c0631f58c9a2f76796)

Co-authored-by: wavebyrd <160968744+wavebyrd@users.noreply.github.com>
Co-authored-by: Carson Jones <carson@Carsons-MacBook-Pro.local>