Fixing incorrect indentation in prometheusRule definitions

Incorrect indentation in severity label in first PrometheusRule definition leads to error when attempting to create the resources:

reconciliation failed: Helm install failed: unable to build kubernetes objects from release manifest: error validating "": error validating data: ValidationError(PrometheusRule.s
pec.groups[0].rules[0]): unknown field "severity" in com.coreos.monitoring.v1.PrometheusRule.spec.groups.rules

Relates #8 (closed)