added exclamation to 00_sum

I added a really super cool exclamation point to 00_sum.  You're going to love it.
In this case, when we do something like `c = Car.new("nissan", "xterra")` we will get the output "NISSAN goes BEEP!".

Instead imagine that we are going to call it like: `c.honk` - how would you do that instead?