Documentation: kvm: fix a typo

Fixes: e287d6de ("Documentation: kvm: Convert cpuid.txt to .rst")
Signed-off-by: Li Qiang <liq3ea@163.com>
Message-Id: <20201001095333.7611-1-liq3ea@163.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>