Documentation/virtual/kvm: Update URL for AMD SEV API specification

The URL of [api-spec] in Documentation/virtual/kvm/amd-memory-encryption.rst
is no longer valid, replaced space with underscore.

Signed-off-by: Christophe de Dinechin <dinechin@redhat.com>
Reviewed-by: Brijesh Singh <brijesh.singh@amd.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>