simple-cipher: remove mention of invalid input from README
There is a corresponding test version update to 1.2.0 but that is not
yet done in this commit... especially since the Rust track is not even
on any 1.x.y version.
@@ -61,10 +61,7 @@ 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 characters in length.
-
-If the key submitted is not composed only of lowercase letters, your
-solution should handle the error in a language-appropriate way.
+at least 100 alphanumeric characters in length.
## Extensions
