[3.12] subprocess docs: Fix semantically important typo (GH-119752) (#119758)

subprocess docs: Fix semantically important typo (GH-119752)

GH-25416 accidentally replaced a reference to the *stderr* argument of
`subprocess.run` with a reference to the *stdin* argument.  *stdin* is
not affected by the `check_output` option.
(cherry picked from commit 2cc3502f98bb9aea386ab55443fc077ddcdde91d)

Co-authored-by: Zachary Ware <zach@python.org>