docs: Fix references to xdm man page

Install xdm man pages under admin section (8), not user programs (1)
Change made on xdm v1.1.12.
https://lists.x.org/archives/xorg-announce/2019-March/002959.html

Reviewed by: dbaio
Pull Request: https://github.com/freebsd/freebsd-doc/pull/52