Fixup path in NetBSD supplied documentation for FreeBSD

NetBSD installs the blacklist-helper script in /libexec, and
it goes into /usr/libexec on FreeBSD. Update the docs to
match FreeBSD's installation location.

Reviewed by: rpaulo
Approved by: rpaulo
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D6592