message: docs: use flatMap instead of map + flat
diff: @@ -30,8 +30,7 @@ Check the [playground](https://github.com/vuejs/vue-router-next/tree/master/play
  - `router.getMatchedComponents` is now removed as they can be retrieved from `router.currentRoute.value.matched`:
  ```js
  router.currentRoute.value.matched
  - .map(record => Object.values(record.components))
  - .flat()
  + .flatMap(record => Object.values(record.components))
  ```
  - If you use a `transition`, you may need to wait for the router to be _ready_ before mounting the app:
  ```js
  