Update the timegm(3) function standard

This updates the standards section of ctime.3 to mention the timegm
function is part of C 23. Would have saved me a bit of reading.

Regards,
--  
Christian


Index: lib/libc/time/ctime.3
===================================================================
RCS file: /cvs/src/lib/libc/time/ctime.3,v
diff -u -p -u -r1.54 ctime.3
--- lib/libc/time/ctime.3	31 May 2026 15:21:04 -0000	1.54
+++ lib/libc/time/ctime.3	20 Jun 2026 05:02:27 -0000
@@ -325,11 +325,14 @@ and
 conform to
 .St -p1003.1-2008 .
 .Pp
-The functions
+The function
 .Fn timegm
-and
+conforms to
+.St -isoC-2023 .
+.Pp
+The function
 .Fn timelocal
-are extensions to these standards.
+is an extension to these standards.
 .Sh HISTORY
 A
 .Fn ctime