tuned function lines_of_atp_problem to have header lines as proper list elements

--- a/src/HOL/Tools/ATP/atp_problem.ML	Wed Dec 18 16:20:34 2024 +0100
+++ b/src/HOL/Tools/ATP/atp_problem.ML	Wed Dec 18 16:32:49 2024 +0100
@@ -801,8 +801,8 @@
   end
 
 fun lines_of_atp_problem format ord_info problem =
-  "% This file was generated by Isabelle (most likely Sledgehammer)\n\
-  \% " ^ timestamp () ^ "\n" ::
+  "% This file was generated by Isabelle (most likely Sledgehammer)" ::
+  timestamp () ::
   (case format of
      DFG poly => dfg_lines poly ord_info
    | _ => tptp_lines format) problem