Update single-application (kiosk) mode topic
https://gitlab.gnome.org/GNOME/gnome-user-docs/-/blob/master/system-admin-guide/C/lockdown-single-app-mode.page (https://help.gnome.org/admin/system-admin-guide/stable/lockdown-single-app-mode.html.en) needs to be updated per feedback provided in https://bugzilla.redhat.com/show_bug.cgi?id=1738792#c5 by @halfline.