docs(SwipeCell): use composition api

src/swipe-cell/README.md

@@ -71,9 +71,11 @@ app.use(SwipeCell);
 ```
 
 ```js
+import { Dialog } from 'vant';
+
 export default {
-  methods: {
-    beforeClose({ position, instance }) {
+  setup() {
+    const beforeClose = ({ position }) => {
       switch (position) {
         case 'left':
         case 'cell':
@@ -81,14 +83,14 @@ export default {
           return true;
         case 'right':
           return new Promise((resolve) => {
-            this.$dialog
-              .confirm({
-                message: 'Are you sure to delete?',
-              })
-              .then(resolve);
+            Dialog.confirm({
+              title: 'Are you sure to delete?',
+            }).then(resolve);
           });
       }
-    },
+    };
+
+    return { beforeClose };
   },
 };
 ```