chore: update example main.dart

Author: LucasXu0

frontend/app_flowy/packages/appflowy_editor/example/lib/main.dart

@@ -3,9 +3,9 @@ import 'dart:convert';
 import 'package:flutter/material.dart';
 import 'package:flutter/services.dart';
 
-import 'package:example/expandable_floating_action_button.dart';
-import 'package:example/plugin/image_node_widget.dart';
-import 'package:example/plugin/youtube_link_node_widget.dart';
+import 'expandable_floating_action_button.dart';
+import 'plugin/image_node_widget.dart';
+import 'plugin/youtube_link_node_widget.dart';
 
 import 'package:appflowy_editor/appflowy_editor.dart';