updated a broken link for newer release notes.

I got a request from Ondrej to fix this: "For up-to-date release notes and errata, see http://www.isc.org/software/ bind9/releasenotes"

We don't have a single URL which will give you release notes for the latest release on every branch, except for the downloads page, which also doesn't have errata. So, i changed it to remove the reference to errata and to point it at the downloads page.