Invalid RBAC example in rbac/v1alpha1/rbac.proto

RBAC example in _rbac/v1alpha1/rbac.proto_ includes

```
//     constraints:
//     - key: "destination.labels[version]"
//       value: ["v1", "v2"]
```

That's not right; it should be `values` not `value.

See also https://github.com/istio/api/issues/847