New review comment by ofiala-a51 on void-packages repository

https://github.com/void-linux/void-packages/pull/24580#discussion_r519427430

Comment:
The group should be prefixed by an underscore (`_seatd`) as [described in Manual.md](https://github.com/void-linux/void-packages/blob/master/Manual.md#creating-system-accountsgroups-at-runtimel)