Documentation: virtual: kvm: correct one bit description in APF case

Documentation: virtual: kvm: correct one bit description in APF case

When commit 6adba5274206 (KVM: Let host know whether the guest can
handle async PF in non-userspace context.) is introduced, actually
bit 2 still is reserved and should be zero. Instead, bit 1 is 1 to
indicate if asynchronous page faults can be injected when vcpu is
in cpl == 0, and also please see this,

in the file kvm_para.h, #define KVM_ASYNC_PF_SEND_ALWAYS (1 << 1).

Signed-off-by: Tiejun Chen <tiejun.chen@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>