at(1): Fix location of at(1) crontab

With r318443, atrun was moved from /etc/crontab to /etc/cron.d/at,
but the man-page was unfortunately not updated to reflect this.

PR: 248048
Submitted by: debdrup
Reported by: yoitsmeremember+fbsd at gmail.com
Reviewed by: Pau Amma <pauamma at gundo.com>
Sponsored by: Klara Inc.
Differential Revision: https://reviews.freebsd.org/D25709