nixos/kmonad: use the official suffix for config file

Both the official Emacs[1] and Vim[2] plugins use kbd as the config file
suffix.

[1]: https://github.com/kmonad/kbd-mode/blob/b07f3e16043fbb268b3e47fb49abc419e8b7e2d7/kbd-mode.el#L275
[2]: https://github.com/kmonad/kmonad-vim/blob/37978445197ab00edeb5b731e9ca90c2b141723f/ftdetect/kbd.vim#L18

nixos/modules/services/hardware/kmonad.nix

@@ -94,7 +94,7 @@ let
       '';
     in
     pkgs.writeTextFile {
-      name = "${mkName keyboard.name}.cfg";
+      name = "${mkName keyboard.name}.kbd";
       text = lib.optionalString keyboard.defcfg.enable (defcfg + "\n") + keyboard.config;
       checkPhase = "${cfg.package}/bin/kmonad -d $out";
     };