Wait for CityStore token for all action types

Thank you for writing this document. I'm just learning Flux and this seemed to be very beneficial.

I changed the final example because I think that `getFlightPriceStore` would need to wait for the `CityStore.dispatchToken` for either action type.
The code can look like this instead:

    switch (payload.actionType) {
      case 'country-update':
      case 'city-update':
        flightDispatcher.waitFor([CityStore.dispatchToken]);
        FlightPriceStore.price =
          getFlightPriceStore(CountryStore.country, CityStore.city);
        break;
    }