message: fix(infinity): change var name
diff: @@ -8,8 +8,8 @@ The ability to inject a infinity load for BetterScroll.
  ```js
  import BScroll from '@better-scroll/core'
  -import Infinity from '@better-scroll/infinity'
  -BScroll.use(Infinity)
  +import InfinityScroll from '@better-scroll/infinity'
  +BScroll.use(InfinityScroll)
  const bs = new BScroll('.wrapper', {
  infinity: {
  