"Compound statement" docs: Fix with-statement step indexing (GH-100286)

Back in commit 226e6e7d4326cf91ef37e13528eb1f62de1bb832
an item was added to the list, renumbering all the rest of the
items, but the forward-reference wasn't updated to match.
(cherry picked from commit f23236a92d8796ae91772adaf27c3485fda963e8)

Co-authored-by: Frank Dana <ferdnyc@gmail.com>