[3.12] gh-101100: Fix Sphinx warnings in `Doc/library/weakref.rst` (GH-109881) (#109884)

[3.12] gh-101100: Fix Sphinx warnings in `Doc/library/weakref.rst` (GH-109881).
(cherry picked from commit 7c61a361fc2e93375e22849fffbc20b60e94dbde)

Co-authored-by: Nikita Sobolev <mail@sobolevn.me>