An updated link for /docs/

[Andras Farkas <deepbluemistake%gmail.com@localhost>]

On
https://www.netbsd.org/docs/
There is a "Single UNIX Specification" link:
http://www.unix.org/online.html

This should probably be supplemented by the somewhat more modern and
useful "POSIX.1":
https://pubs.opengroup.org/onlinepubs/9699919799/
This is simply a more modern version of what's already linked, and far
more importantly does *not* require registering or logging in to read
the spec.