[3.12] Normalize heading underline in multiprocessing.rst (GH-114923) (#114946)

This gets rid of the mildly confusing `>>>>>>>' underlines which look vaguely like `diff` punctuation.
(cherry picked from commit 00d7109075dfaadf438362c084e8a1890c53d4c8)

Co-authored-by: Skip Montanaro <skip.montanaro@gmail.com>