docs（button）: remove useless code

Author: 2534290808

lib/button.dart

@@ -1,6 +1,5 @@
 import 'package:flutter/material.dart';
 import 'package:smart_color/smart_color.dart';
-import 'package:flutter/services.dart';
 import 'package:flutter/cupertino.dart';
 
 ///按钮的三种类型