[tz] POSIX.1-2024 link in theory.html

Hi, I have noticed that <https://data.iana.org/time-zones/theory.html> states

As of this writing, the current edition of POSIX is POSIX.1-2024, which has been published but not yet in HTML form.

It seems, it is time to update this paragraph. For precise location in the repository see the attached file. I am unsure if a more precise link https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/V1_chap08.html#tag... should be used. Perhaps some other links should be updated to point to the newer standard revision.