[3.14] gh-138092: Correct the documented signature of ``mmap.flush`` (GH-138671) (#139202)

gh-138092: Correct the documented signature of ``mmap.flush`` (GH-138671)
(cherry picked from commit e101f907dc827e3dc0bd3b08e4a0897d4a467430)

Co-authored-by: AN Long <aisk@users.noreply.github.com>