Scoped CSS documentation should be updated to use data property

I'm starting out with Vue 2.0 and I just noticed that the [scoped documentation](https://github.com/vuejs/vue-loader/blob/master/docs/en/features/scoped-css.md) is adding a normal valueless property to the element for the scoped CSS but in the latest 2.0 code it is adding a data attribute instead.
