net/xrdp: Use canonical spelling of USERS/GROUPS

When the _xrdp user and group were added, they were listed as
USER= _xrdp
GROUP= _xrdp
which did not result in the user and group being created when the
package is installed.

Reviewed by: meta
Fixes: ee46045d223a ("net/xrdp: Update to 0.10.2-rc.1")
Sponsored by: Amazon
Differential Revision: https://reviews.freebsd.org/D59324

(cherry picked from commit 6e9e6da50fe9a5b1d4260a57a041e68b72694b6a)