Fix documentation links to the vorbis website.

Fix documentation links to the vorbis website.

The server is using .shtml index pages, so linking directly
to an index.html page does not work. Truncate the reference
at the directory path and rely on the server's default page
to select the correct contents.

Thanks to Mark Harris for pointing out the breakage.

Signed-off-by: Mark Harris <mark.hsj@gmail.com>