test_inject_will_blow_your_mind uses an incorrect memo argument name

No big NBD, but the second memo argument is better called 'product' than 'sum'.
Merge branch 'patch-1' of https://github.com/backspace/ruby_koans into product

* 'patch-1' of https://github.com/backspace/ruby_koans:
  test_inject_will_blow_your_mind uses an incorrect memo argument name