Fix missing dash in flag for statsd container

[olchas]

This PR fixes a bug in the definition of statsd container in helm-chart.

With current master helm-chart the airflow-statsd pod fails and collecting logs from the pod with kubectl logs returns the message:

statsd_exporter: error: unknown short flag '-s', try --help

Following the suggestion from prometheus/statsd_exporter#155, I have added additional dash to -statsd.mapping-config flag in the definition of statsd container inside statsd-deployment.yaml file, which seems to fix the pod.

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