nixos/gdm: Fix tmpfiles configuration syntax

Fixes this error from `nixos-rebuild switch` introduced by #75893:

setting up tmpfiles
[/etc/tmpfiles.d/nixos.conf:7] Invalid age 'yes'.
warning: error(s) occurred while switching to the new configuration

Signed-off-by: Anders Kaseorg <andersk@mit.edu>

nixos/modules/services/x11/display-managers/gdm.nix

@@ -171,7 +171,7 @@ in
       "L+ /run/gdm/.config/pulse - - - - ${pulseConfig}"
     ] ++ optionals config.services.gnome3.gnome-initial-setup.enable [
       # Create stamp file for gnome-initial-setup to prevent it starting in GDM.
-      "f /run/gdm/.config/gnome-initial-setup-done 0711 gdm gdm yes"
+      "f /run/gdm/.config/gnome-initial-setup-done 0711 gdm gdm - yes"
     ];
 
     systemd.services.display-manager.wants = [