[3.13] gh-101100: Fix Sphinx warnings in `Doc/library/signal.rst` (GH-139930) (GH-139967)

gh-101100: Fix Sphinx warnings in `Doc/library/signal.rst` (GH-139930)
(cherry picked from commit 447c7a89fb41b7fa84b9b26f111aedd649bc5400)

Co-authored-by: Weilin Du <108666168+LamentXU123@users.noreply.github.com>