user's guesses are checked and marked as complete

I think the service object might be overkill. (This update method seems long as well). How about something like this?

```ruby
def update
  if guessed_correctly?
    guess.correct = true
  end

  guess.mark_complete
  set_flash_message
end

def guessed_correctly?
  @_guessed_correctly ||= guess.correct?(answer)
end
```