[3.11] Fix typos in ElementTree documentation (GH-108848) (GH-114855)

PI objects instead of comment objects.
(cherry picked from commit de6f97cd3519c5d8528d8ca1bb00fce4e9969671)

Co-authored-by: Christophe Nanteuil <35002064+christopheNan@users.noreply.github.com>