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

[miss-islington]

Back in commit 226e6e7
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 f23236a)

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

[miss-islington]

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[miss-islington]

Sorry, I can't merge this PR. Reason: You're not authorized to push to this branch. Visit https://docs.github.com/articles/about-protected-branches/ for more information..