Update snippet05.ml to match Haskell snippet

This code is the same code found in the KleisliIdentity module in section 8.5 in order to match the Haskell code to which it is associated

Author: jakubsvec001

src/content/1.4/code/ocaml/snippet05.ml

@@ -1,3 +1 @@
-let up_case : string -> string writer =
- fun s -> String.uppercase s, "up_case "
-;;
+let return : 'a -> 'a writer = fun a -> a, ""