updated project style guidelines

lib/api/mhsl/customTimetableEvent/add/addCustomTimetableEvent.dart

@@ -16,7 +16,7 @@ class AddCustomTimetableEvent extends MhslApi<void> {
 
   @override
   Future<Response>? request(Uri uri) {
-    String body = jsonEncode(params.toJson());
+    var body = jsonEncode(params.toJson());
     return http.post(uri, body: body);
   }
-}
+}