Documentation: KVM: Update AMD memory encryption link

Update AMD memory encryption white-paper document link.
Previous link is not available. Update new available link.

Signed-off-by: Wyes Karny <wyes.karny@amd.com>
Reviewed-by: Carlos Bilbao <carlos.bilbao@amd.com>
Link: https://lore.kernel.org/r/20230125175948.21100-1-wyes.karny@amd.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>