brightnessKey: Set step size to 5

modules/services/backlight.nix

@@ -2,7 +2,10 @@
   flake.modules.nixos.backlight = {
     programs.light = {
       enable = true;
-      brightnessKeys.enable = true;
+      brightnessKeys = {
+        enable = true;
+        step = 5;
+      };
     };
   };
 }