[3.13] [docs] fix a Sphinx directive in `c-api/object.rst` (GH-121430) (#121440)

[docs] fix a Sphinx directive in `c-api/object.rst` (GH-121430)
(cherry picked from commit ada964fba06c0d142b8746d140817ea151314d33)

Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>