Fix Introduction in getting started

[SCullman]

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

[dbrattli]

Thanks. I will fix more of the docs later today.