[3.12] gh-123110: correct note about _Bool in the struct module docs (GH-123111) (#123127)

gh-123110: correct note about _Bool in the struct module docs (GH-123111)
(cherry picked from commit b0f462d4c808d6fb1d381bba4932acd8309c1f3b)

Co-authored-by: Sergey B Kirpichev <skirpichev@gmail.com>