[docs] Update Flux TodoMVC Tutorial

I was reminded by @whatasunnyday in the Flux repo's [issue #37](https://github.com/facebook/flux/issues/37) that the Flux TodoMVC tutorial needs to be updated to reflect the latest dispatcher we have released.  The tutorial had previously used a dispatcher based on promises, and this was confusing for newcomers.
(when do and don't you need an AppDispatcher?)