gh-62480: De-personalize "Mocking Unbound Methods" section in `unittest.mock` examples (#141322)

* Rewrite Mocking Unbound Methods paragraph to second person

Co-authored-by: C.A.M. Gerlach <CAM.Gerlach@Gerlach.CAM>
Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>