KVM: Documentation: Fix index for KVM_CAP_PPC_DAWR1

It should be 7.23 instead of 7.22, which has already been taken by
KVM_CAP_X86_BUS_LOCK_EXIT.

Signed-off-by: Kai Huang <kai.huang@intel.com>
Message-Id: <20210226094832.380394-1-kai.huang@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>