Added array substraction koans to about_arrays.rb

Hi, this is Raul Garcia from Spain, I'm using ruby koans to learn and practice some ruby and they are great!, but I had some issues when I was finishing them and I think this modification to the about_array.rb file will be very useful to new students like me.

I have added some new koans to the file in order to explain how array substraction really works because it may be confusing for new students due to the similarities with the fixnum substraction instruction.

I hope that you find this useful too.
Raúl García