Reverse the order of queue and on_complete in the third Hydra example.

Revise the third Hydra example to reverse the order of the queue and callback definition. While not a problem with the code in its current form, I believe that if these two lines were reused in a more complex example they could result in an (albeit unlikely) race condition where the request could complete before the callback is defined.

Consider this example (an amalgamation of the first example and the third in its current form (third request queued before its callback is defined):

```ruby
hydra = Typhoeus::Hydra.hydra

first_request = Typhoeus::Request.new("www.example.com/posts/1.json")
first_request.on_complete do |response|
  third_request = Typhoeus::Request.new("www.example.com/posts/3.json")
  hydra.queue third_request
  third_request.on_complete do |response|
    # do_something_with response
  end
end
second_request = Typhoeus::Request.new("www.example.com/posts/2.json")

hydra.queue first_request
hydra.queue second_request
# this is a blocking call that returns once all requests are complete
hydra.run
```
Because when the third request is queued hydra is already running, I'm assuming a race could occur. As I submitted this third example, and as the order of queue and callback was opposite to other examples, I thought it best to correct it.
Merge pull request #405 from mbrookes/patch-3

Reverse the order of queue and on_complete in the third Hydra example.