The flag stated in the example readme is `--default-backend` but passing that (or looking at --help`) for the ingress controller shows:
unknown flag: --default-backend
...
--default-backend-service string Service used to serve HTTP requests not matching any known server name (catch-all).
Takes the form "namespace/name". The controller configures NGINX to forward
requests to the first port of this Service.
ref: https://github.com/kubernetes/ingress-nginx/blob/master/cmd/nginx/flags.go#L55
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