net/xrdp-devel: Use canonical spelling of USERS/GROUPS

Applying D59324 to net/xrdp-devel as well.

Reported by: cperciva
Fixes: df5ed26f9502 ("net/xrdp-devel: Update to 0.10.80.b20241229 4c8c773")
Sponsored by: Amazon
Sponsored by: Cybertrust Japan
Differential Revision: https://reviews.freebsd.org/D59324