Update description.md (exercism#1653)

The test suite is testing against the key having only lowercase letters, not alphanumeric.

exercises/simple-cipher/description.md

@@ -59,7 +59,7 @@ substitution cipher a little more fault tolerant by providing a source
 of randomness and ensuring that the key contains only lowercase letters.
 
 If someone doesn't submit a key at all, generate a truly random key of
-at least 100 alphanumeric characters in length.
+at least 100 lowercase characters in length.
 
 ## Extensions