docs: kvm: fix referenced ioctl symbol

docs: kvm: fix referenced ioctl symbol

The actual symbol that is exported and usable is
'KVM_MEMORY_ENCRYPT_OP', not 'KVM_MEM_ENCRYPT_OP'

$ git grep -l KVM_MEM_ENCRYPT_OP
Documentation/virt/kvm/amd-memory-encryption.rst

$ git grep -l KVM_MEMORY_ENCRYPT_OP
Documentation/virt/kvm/api.rst
arch/x86/kvm/x86.c
include/uapi/linux/kvm.h
tools/include/uapi/linux/kvm.h

While we're in there, update the KVM API category for
KVM_MEMORY_ENCRYPT_OP. It is called on a VM file descriptor.

Signed-off-by: Connor Kuehl <ckuehl@redhat.com>
Link: https://lore.kernel.org/r/20200819211952.251984-1-ckuehl@redhat.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>