message: docs(observedom): fix typo
diff: @@ -18,11 +18,11 @@ yarn add @better-scroll/observe-dom@next
  ```js
  import BScroll from '@better-scroll/core'
  - import ObserveDom from '@better-scroll/observe-dom'
  - BScroll.use(ObserveDom)
  + import ObserveDOM from '@better-scroll/observe-dom'
  + BScroll.use(ObserveDOM)
  new BScroll('.bs-wrap', {
  //...
  - observeDom: true // set observeDom to true
  + observeDOM: true // set observeDOM to true
  })
  ```
  