message: docs(ImagePreview): update demo
diff: @@ -20,7 +20,7 @@ ImagePreview([
  ]);
  ```
  -### Custom config
  +### Set Start Position
  ```js
  ImagePreview({
  @@ -29,9 +29,6 @@ ImagePreview({
  'https://img.yzcdn.cn/vant/apple-2.jpg',
  ],
  startPosition: 1,
  - onClose() {
  - // do something
  - },
  });
  ```
  @@ -49,6 +46,22 @@ ImagePreview({
  });
  ```
  +### Close Event
  +
  +```js
  +import { Toast } from 'vant';
  +
  +ImagePreview({
  + images: [
  + 'https://img.yzcdn.cn/vant/apple-1.jpg',
  + 'https://img.yzcdn.cn/vant/apple-2.jpg',
  + ],
  + onClose() {
  + Toast('closed');
  + },
  +});
  +```
  +
  ### Async Close
  ```js
  @@ -62,7 +75,7 @@ const instance = ImagePreview({
  setTimeout(() => {
  instance.close();
  -}, 1000);
  +}, 2000);
  ```
  ### Component Call
  @@ -85,7 +98,6 @@ export default {
  ],
  };
  },
  -
  methods: {
  onChange(index) {
  this.index = index;
  