bitset: Fix __BIT_FFS_AT()'s herald comment

The manual page is correct though.

No functional change.

Reviewed by: emaste
Approved by: emaste (mentor)
MFC after: 3 days
Event: Kitchener-Waterloo Hackathon 202406
Sponsored by: The FreeBSD Foundation

(cherry picked from commit f3ab0d86e8070c73bf2e2852341f8e5c709f5682)

Approved by: emaste (mentor)