fix a typo in the error message of refernces (BUG#45)

typing/typer.ml

@@ -1456,7 +1456,7 @@ substtitution: %a @]@\n%!"
     if not(Var.Set.subset vars env.delta) then
       error loc (
         Printf.sprintf
-          "Type ref %s constains polymorphic variables"
+          "Type ref (%s) contains polymorphic variables"
           (Types.Print.string_of_type td))
     else begin
       ignore (type_check env e td false);