Listen command help doesn't show an example with custom address

[leetrout]

Currently the help output for the listen command is:

$ hookdeck listen -h
Forward webhooks for a source to your local server

Usage:
  hookdeck listen [flags]

Flags:
  -h, --help   help for listen

Global Flags:
      --cli-key string        Your CLI key to use for the command
      --color string          turn on/off color output (on, off, auto)
      --config string         config file (default is $HOME/.config/hookdeck/config.toml)
      --device-name string    device name
      --log-level string      log level (debug, info, warn, error) (default "info")
  -p, --project-name string   the project name to read from for config (default "default")

If you run listen with no arguments:

$ hookdeck listen
Requires a port or forwarding URL to foward the webhooks to

We should change the description to include a reference to the required arguments as examples

hookdeck listen [flags] <hostname>[:port]