[3.14] gh-138659: Typo in the gc module docstring (GH-138660) (#138662)

gh-138659: Typo in the gc module docstring (GH-138660)

docs(gc): fix typo in get_threshold() docstring

Removes a duplicate "the" from the docstring for the `gc.get_threshold()` function.
(cherry picked from commit c006a623e73d2368ec653e19c769706885161053)

Co-authored-by: Aalaap Dey <65075436+axdeyy@users.noreply.github.com>