Update snippet04.ml to match Haskell snippet

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

Author: jakubsvec001

src/content/1.4/code/ocaml/snippet04.ml

@@ -1 +1,4 @@
-let pure x = x, ""
+let ( >=> ) = fun m1 m2 ->
+  let y, s1 = m1 x in
+  let z, s2 = m2 x in
+    z, StringLabels.concat +sep:"" [ s1; s2 ]