[3.11] GH-82695: Clarify `pathlib.Path.mkdir()` documentation (GH-114032) (#114463)

Remove a double negative in the documentation of `mkdir()`'s *exist_ok*
parameter.

(cherry picked from commit 32c227470aa6f72950b76206ffc529c258b4b8fa)

Co-authored-by: Barney Gale <barney.gale@gmail.com>
Co-authored-by: Adam Turner <9087854+AA-Turner@users.noreply.github.com>