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

Documentation/virt/kvm/api.rst

@@ -6360,7 +6360,7 @@ system fingerprint.  To prevent userspace from circumventing such restrictions
 by running an enclave in a VM, KVM prevents access to privileged attributes by
 default.
 
-See Documentation/x86/sgx/2.Kernel-internals.rst for more details.
+See Documentation/x86/sgx.rst for more details.
 
 8. Other capabilities.
 ======================