Fix a link typo in the spec colophon.
This was introduced by the recent https link update patch.
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/'}{About
+everyone honest. See \href{https://xiph.org/about/}{About
the Xiph.Org Foundation} for details.
