x86/sev/docs: Update document URL in amd-memory-encryption.rst

The previous link to AMD programmer's manual is no longer available,
replace it with the new one.

Signed-off-by: Jianlin Li <ljianlin99@gmail.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20230922082547.522689-1-ljianlin99@gmail.com