Removing the link to the wiki as discussed with @bsdphk.

The link for the Varnish book refers to the old one about Varnish 4. A new one has been released a couple of months ago which is about Varnish 6.

Fix an old obvious failure on my part

The 'Changes' and 'Upgrading' links are 6.6/6.5 general, not 6.6.1/6.5.2
specific

6.4 has passed its EOL date (2021-03), so it should not be listed in the
top release table any longer.

Fixes: #94

Thank you to Martin