docs: fix cdn link (#8023)

src/form/README.md

@@ -286,7 +286,7 @@ import { ref } from 'vue';
 
 export default {
   setup() {
-    const value = ref([{ url: 'https://img.yzcdn.cn/vant/leaf.jpg' }]);
+    const value = ref([{ url: 'https://img01.yzcdn.cn/vant/leaf.jpg' }]);
     return { value };
   },
 };