gh-57762: fix misleading tkinter.Tk docstring (GH-98837)

Mentioned as a desired change by terryjreedy on the corresponding issue,
since Tk is not a subclass of Toplevel.
(cherry picked from commit ad23da0e77c4c4a3185df64d1c199b8c17e08188)

Co-authored-by: Shantanu <12621235+hauntsaninja@users.noreply.github.com>