ServiceMonitor selector label default value update

[PeiyuanQi]

In The chart 0.1.12 values.yaml file, tehere is a field:
prometheus.serviceMonitor.selector.prometheus: kube-prometheus
The comment is not clear about the usage of this field and also no documentation about. This field should be the value from
kubectl get prometheus -o yaml -n <prometheus namespace> serviceMonitorSelector.matchLabels.

In the case prometheus-operator, the value should be release: prometheus-operator.

Then the servicemonitor could be discovered by prometheus in targets