cron: Fix comment from "root's" to "system" crontab

It is referred to in the documentation as the system crontab, not root's
crontab.

PR: 289099
Reviewed by: emaste
Approved by: emaste (mentor)
Fixes: d33daab9348a ("Call /etc/crontab the "system crontab", not "root's crontab". While here, fix some other wording issues")
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D52503

(cherry picked from commit c806defe8fee62c4738f46e555d69b085e8d1259)