Pre Merge pull request !2 from 崔明辉/feature/useRect

packages/vant/docs/markdown/use-rect.zh-CN.md

@@ -13,15 +13,18 @@
 ```
 
 ```js
-import { ref } from 'vue';
+import { ref,onMounted } from 'vue';
 import { useRect } from '@vant/use';
 
 export default {
   setup() {
     const root = ref();
-    const rect = useRect(root);
 
+    onMounted(()=>{
+      const rect = useRect(root);
       console.log(rect); // -> 元素的大小及其相对于视口的位置
+    })
+    
 
     return { root };
   },