Change CLI help message for adding gateway collaborators

This is a cosmetic bug report. for ttnctl.

What do you want to do?

Add a collaborator to a gateway using the CLI.

What steps did you take?

$ ttnctl gateways collaborators add --help (Not the best methods to get help i agree)

What went wrong or what is missing?

Usage:
  ttnctl gateways collaborators add [AppID] [Username] [Rights...] [flags]

The description must say [GatewayID] instead of [AppID]. Though this is sort of obvious, it might confuse new users.

Do you have Screenshots?

What kind of OS/Browser/Gateway are you using? Which version?

This is applicable to all.

What are the IDs and EUIs of your Device/Gateway? (if applicable)

What do your configuration files look like?

What do your log files look like?

Can you fix this yourself and submit a pull request?

Yes.

TheThingsArchive / ttn