Fix about link in the specification.
The old .html versions of these pages no longer redirect to
to the new-style directory-based urls, so this link was
broken.
Ogg is a \href{https://xiph.org/}{Xiph.Org Foundation} effort
to protect essential tenets of Internet multimedia from corporate
hostage-taking; Open Source is the net's greatest tool to keep
-everyone honest. See \href{https://xiph.org/about.html}{About
+everyone honest. See \href{https://xiph.org/about/'}{About
the Xiph.Org Foundation} for details.
