docs: virt: api.rst: fix a pointer to SGX documentation

The document which describes the SGX kernel architecture was added at
commit 3fa97bf0

 ("Documentation/x86: Document SGX kernel architecture")

but the reference at virt/kvm/api.rst is pointing to some
non-existing document.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/138c24633c6e4edf862a2b4d77033c603fc10406.1621413933.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>