roles/openshift_metering: Fix typo in readme

roles/openshift_metering/README.md

@@ -37,7 +37,7 @@ Updating the operator itself to a custom image can be done by setting `openshift
 For example:
 
 ```
-openshift_metering_config: quay.io/coreos/chargeback-helm-operator:latest
+openshift_metering_operator_image: quay.io/coreos/chargeback-helm-operator:latest
 ```
 
 Using a custom project/namespace can be done by specifying `__openshift_metering_namespace`.