Fix a type: s/is/is a/

Co-authored-by: András B Nagy <[email protected]>

Author: IsaacG

exercises/simple-cipher/description.md

@@ -1,7 +1,7 @@
 # Description
 
 Create an implementation the [Vigenère cipher][wiki].
-The Vigenère cipher is simple substitution cipher.
+The Vigenère cipher is a simple substitution cipher.
 
 ## Cipher terminology