message: docs(CountDown): use composition api
diff: @@ -19,11 +19,12 @@ app.use(CountDown);
  ```
  ```js
  +import { ref } from 'vue';
  +
  export default {
  - data() {
  - return {
  - time: 30 * 60 * 60 * 1000,
  - };
  + setup() {
  + const time = ref(30 * 60 * 60 * 1000);
  + return { time };
  },
  };
  ```
  @@ -79,7 +80,7 @@ export default {
  :time="3000"
  :auto-start="false"
  format="ss:SSS"
  - @finish="finish"
  + @finish="onFinish"
  />
  <van-grid clickable :column-num="3">
  <van-grid-item text="Start" icon="play-circle-o" @click="start" />
  @@ -92,19 +93,29 @@ export default {
  import { Toast } from 'vant';
  export default {
  - methods: {
  - start() {
  - this.$refs.countDown.start();
  - },
  - pause() {
  - this.$refs.countDown.pause();
  - },
  - reset() {
  - this.$refs.countDown.reset();
  - },
  - finish() {
  + setup() {
  + const countDown = ref(null);
  +
  + const start = () => {
  + countDown.value.start();
  + };
  + const pause = () => {
  + countDown.value.pause();
  + };
  + const reset = () => {
  + countDown.value.reset();
  + };
  + const onFinish = () => {
  Toast('Finished');
  - },
  + };
  +
  + return {
  + start,
  + pause,
  + reset,
  + onFinish,
  + countDown,
  + };
  },
  };
  ```
  