Update the first Hydra example for clarity

 Revised the first parallel request example code (based on a previous version) to clarify that the third request is run in the context of the first request, and so is dependant on the result of the first request before it can be run, explaining the use of the callback.

Also reversed the order of the callback and queue in the third example, as the current code, if used in a callback to add a request to an already running queue, could result in a theoretical (if unlikely) race condition.
I don't mind changing this, but I don't think there is a race condition because `hydra.run` is called after the map.