gh-131740: minor readability fix in PyUnstable_GC_VisitObjects (gh-131786)

Minor readability fix in PyUnstable_GC_VisitObjects

Replaces `if (visit_generation())` with `if (visit_generation() < 0)`,
since we are checking for the failure case, and it's confusing to have
that be implicitly `true`.

Also fixes a misspelt variable name.