tuned formatting of error message

--- a/src/Tools/adhoc_overloading.ML	Wed Aug 07 14:47:50 2013 +0200
+++ b/src/Tools/adhoc_overloading.ML	Fri Aug 02 12:11:02 2013 +0900
@@ -41,7 +41,7 @@
       "in term " ::
       quote (Syntax.string_of_term ctxt' t) ::
       (if null instances then "no instances" else "multiple instances:") ::
-    map (Syntax.string_of_term ctxt') instances)
+    map (Markup.markup Markup.item o Syntax.string_of_term ctxt') instances)
     |> error
   end;