message: docs: update cdn link to 2.1
diff: @@ -95,11 +95,11 @@ Vue.use(Vant);
  ```html
  <!-- import style -->
  -<link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/vant@2.0/lib/index.css" />
  +<link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/vant@2.1/lib/index.css" />
  <!-- import script -->
  <script src="https://cdn.jsdelivr.net/npm/vue/dist/vue.min.js"></script>
  -<script src="https://cdn.jsdelivr.net/npm/vant@2.0/lib/vant.min.js"></script>
  +<script src="https://cdn.jsdelivr.net/npm/vant@2.1/lib/vant.min.js"></script>
  <script>
  var Vue = window.Vue;
  