Updating atbash-cipher instructions

Exercism Building Exercism
1

First line in specs reads ‘atbash’, while the second reads ‘Atbash’. I propose:

  1. Change both to ‘Atbash’.
  2. Add a link to its Wikipedia page Atbash - Wikipedia

Edit: I now see a link to Wikipedia that comes from metadata.toml. Disregard the second suggestion.