fix Time.to_sec_string example again

`Time.Zone.local` has been removed from the package `core`. The current
version, 0.16.1, has a deprecation warning in its place. There is a
platform-specific replacement in `core-unix`. To avoid requiring
`core-unix`, let's change the time zone to UTC.

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 Time.Zone.local ^ " -- " ^ message
+    Time.to_sec_string time ~zone:Time.Zone.utc ^ " -- " ^ message
   ;;
 log_entry (Some Time.epoch) "A long long time ago";;
 log_entry None "Up to the minute";;