nixos/klipper: Add default value for apiSocket

nixos/modules/services/misc/klipper.nix

···
 
       apiSocket = mkOption {
         type = types.nullOr types.path;
-        default = null;
-        example = "/run/klipper/api";
+        default = "/run/klipper/api";
         description = "Path of the API socket to create.";
       };