KVM: Documentation: Fix KVM_CAP_ENFORCE_PV_FEATURE_CPUID name

'KVM_CAP_ENFORCE_PV_CPUID' doesn't match the define in
include/uapi/linux/kvm.h.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Message-Id: <20210722092628.236474-1-vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>