Fix mutating webhook default state in README.md (#993)
Fix mutating webhook default state in README.md (#993)

The readme indicated that the mutating webhook was enabled by default
when spark-operator is installed via helm. This was inconsistent with
other documentation and the default value of `enableWebhook` in the
chart's values.yaml[1].

[1] https://github.com/helm/charts/blob/709c01f6e2fadb49d23cc80c62d1d5a169d0c938/incubator/sparkoperator/values.yaml#L26
GoogleCloudPlatform/spark-on-k8s-operator,GoogleCloudPlatform/spark-on-k8s-operator