__FreeBSD_version: update the references to the doc tree

Update the reference of which file to update in the doc tree when
bumping __FreeBSD_version.

This change is to catch up with commit f8fed61b80 in the doc repository.

MFC after: 3 days
Approved by: lwhsu (mentor)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D29920