Fix Time.to_sec_string example

As of Core version 112.17.00 Time.to_sec_string requires a time zone.

Author: dbohdan

code/guided-tour/main.topscript

@@ -147,7 +147,7 @@ let log_entry maybe_time message =
       | Some x -> x
       | None -> Time.now ()
     in
-    Time.to_sec_string time ^ " -- " ^ message
+    Time.to_sec_string Time.Zone.local time ^ " -- " ^ message
   ;;
 log_entry (Some Time.epoch) "A long long time ago";;
 log_entry None "Up to the minute";;