message: docs(ImagePreview): fix root attr
diff: <template>
  <demo-section>
  <demo-block card :title="t('basicUsage')">
  - <van-cell is-link @click="showImagePreview">
  + <van-cell is-link @click="showImagePreview()">
  {{ t('showImages') }}
  </van-cell>
  </demo-block>
  @@ -104,7 +104,7 @@ export default {
  this.index = index;
  },
  - showImagePreview(options) {
  + showImagePreview(options = {}) {
  const instance = ImagePreview({
  images,
  ...options,
  