atbash-cipher: update description to be explicit about lowercase encoding (#2068)

The test suite expects uppercase letters to be encoded as lowercase. Neither the instructions nor the examples address this.

Author: dsletten

exercises/atbash-cipher/description.md

@@ -21,6 +21,7 @@ been an issue in the cipher's time.
 Ciphertext is written out in groups of fixed length, the traditional group size
 being 5 letters, leaving numbers unchanged, and punctuation is excluded.
 This is to make it harder to guess things based on word boundaries.
+All text will be encoded as lowercase letters.
 
 ## Examples