Change parameter name in helm's values file

The name `apiServer` in https://github.com/microsoft/retina/blob/57caa931edfbadc4d923ae74b47a3ccc8b82c8c6/deploy/hubble/manifests/controller/helm/retina/values.yaml#L77-L79 is misleading as it could lead one to think this configuration is about kubeapi server, which is not.