[3.6] bpo-31621: Fix typo in Simple Statements documentation (GH-3809)

[miss-islington]

Replace "restriction" with "restrictions".
(cherry picked from commit 8d59aca)

https://bugs.python.org/issue31621

[the-knights-who-say-ni]

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[miss-islington]

@kms70847 and @Mariatta: Backport status check is done, and it's a success ✅ .

[miss-islington]

Thanks, @Mariatta!