Fix Introduction in getting started

> To use Fable Reaction with Elmish you need to call the Program.withStream with your reactive query. The query function takes an IAsyncObservable<'msg> and returns a possibibly transformed IAsyncObservable<'msg>.

This doesn't fit to the following code example in [Getting Started](https://fablereaction.readthedocs.io/en/latest/fable.reaction/getting_started.html)