Fix markdown comment for linter

From the different options at: https://stackoverflow.com/questions/4823468/comments-in-markdown

exercises/affine-cipher/description.md

@@ -6,7 +6,7 @@ The affine cipher is a type of monoalphabetic substitution cipher.
 Each character is mapped to its numeric equivalent, encrypted with a mathematical function and then converted to the letter relating to its new numeric value.
 Although all monoalphabetic ciphers are weak, the affine cipher is much stronger than the atbash cipher, because it has many more keys.
 
-[comment]: # ( monoalphabetic as spelled by Merriam-Webster, compare to polyalphabetic )
+[//]: # ( monoalphabetic as spelled by Merriam-Webster, compare to polyalphabetic )
 
 ## Encryption