loader(8): fix path to be correct loader.4th.

boot.4th was a thing for only a few months around FreeBSD 3.1. The
correct name has been loader.4th for a long time.

MFC After: 2 days
Sponsored by: Netflix

(cherry picked from commit 13c98cd06b80afd0d062211ee8f1bfeb8c6cb065)