net/xrdp: Use canonical spelling of USERS/GROUPS

[cperciva]

Summary

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.

Fixes: ee46045d223a ("net/xrdp: Update to 0.10.2-rc.1")
Sponsored by: Amazon

[meta]

Thanks! Feel free to commit it.