corrected variable renaming

--- a/src/Pure/Isar/code_unit.ML	Mon Mar 23 19:01:16 2009 +0100
+++ b/src/Pure/Isar/code_unit.ML	Mon Mar 23 19:01:17 2009 +0100
@@ -218,7 +218,7 @@
     |> burrow_thms (canonical_tvars thy purify_tvar)
     |> map (canonical_vars thy purify_var)
     |> map (canonical_absvars purify_var)
-    |> map Drule.zero_var_indexes
+    |> Drule.zero_var_indexes_list
   end;