docs: prefer shorter demo

src/swipe/README.zh-CN.md

@@ -80,9 +80,7 @@ import { Toast } from 'vant';
 
 export default {
   setup() {
-    const onChange = (index) => {
-      Toast('当前 Swipe 索引：' + index);
-    };
+    const onChange = (index) => Toast('当前 Swipe 索引：' + index);
     return { onChange };
   },
 };