fix(graindoc): Enable printing of arrow types (#1367)

Author: ospencer

compiler/src/typed/oprint.re

@@ -651,8 +651,8 @@ and print_out_sig_item = ppf =>
           failwith("NYI: Otyp_variant pretty-printer")
         | Otyp_open => failwith("NYI: Otyp_open pretty-printer")
         | Otyp_alias(_, _) => failwith("NYI: Otyp_alias pretty-printer")
-        | Otyp_arrow(_, _) => failwith("NYI: Otyp_arrow pretty-printer")
         | Otyp_class(_, _, _) => failwith("NYI: Otyp_class pretty-printer")
+        | Otyp_arrow(_, _)
         | Otyp_abstract
         | Otyp_tuple(_)
         | Otyp_constr(_, _) => "type"