flutter: main_menu.dart 改为 UTF-8 with BOM 格式

src/ui/flutter/lib/widgets/main_menu.dart

@@ -1,4 +1,4 @@
-/*
+/*
   FlutterMill, a mill game playing frontend derived from ChessRoad
   Copyright (C) 2019 He Zhaoyun (ChessRoad author)
   Copyright (C) 2019-2020 Calcitem <calcitem@outlook.com>
@@ -136,14 +136,14 @@ class _MainMenuState extends State<MainMenu> with TickerProviderStateMixin {
           ),
           Expanded(child: SizedBox()),
           Text(
-              '              ??????\n'
-              '??????,???????\n'
-              '??????,???????\n'
-              '??????,???????\n'
-              '??????,???????',
+              '              健康游戏忠告\n'
+              '抵制不良游戏，拒绝盗版游戏。\n'
+              '注意自我保护，谨防受骗上当。\n'
+              '适度游戏益脑，沉迷游戏伤身。\n'
+              '合理安排时间，享受健康生活。',
               style: TextStyle(color: Colors.black54, fontSize: 16)),
           Expanded(child: SizedBox()),
-          Text('Copyright © 2019-2020 The Sanmill Authors',
+          Text('Copyright © 2019-2020 The Sanmill Authors',
               style: TextStyle(color: Colors.black54, fontSize: 16)),
           Expanded(child: SizedBox()),
         ],