docs: kvm: x86: Fix broken field list

Add a missing ":" to fix a broken field list.

Signed-off-by: Takahiro Itazuri <itazur@amazon.com>
Fixes: ba7bb663 ("KVM: x86: Provide per VM capability for disabling PMU virtualization")
Message-Id: <20230331093116.99820-1-itazur@amazon.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>