docs: modify it to the correct example link. (#2390)

The link is broken. However, the one I modified may not be the best, but
at least it is correct.

Author: StanleyCocos

dio/example/dio.dart

@@ -1,6 +1,6 @@
 import 'package:dio/dio.dart';
 
-/// More examples see https://github.com/cfug/dio/blob/main/example
+/// More examples see https://github.com/cfug/dio/tree/main/dio#examples
 void main() async {
   final dio = Dio();
   final response = await dio.get('https://pub.dev');