Mention `--backend` flag in cortextool help command

[eamonryan]

Currently the help command on version 0.10.7 says:

Flags:
  --help                      Show context-sensitive help (also try --help-long and
                              --help-man).
  --log.level="info"          set level of the logger
  --push-gateway.endpoint=PUSH-GATEWAY.ENDPOINT  
                              url for the push-gateway to register metrics
  --push-gateway.job=PUSH-GATEWAY.JOB  
                              job name to register metrics
  --push-gateway.interval=1m  interval to forward metrics to the push gateway

There is no mention of the --backend flag, despite the fact that it exists and is used for example when interacting with a Loki ruler as written here: https://grafana.com/docs/loki/latest/rules/#interacting-with-the-ruler

It would be helpful to add mention of the flag to the help command.