Fix typo in domains subcommand listing

[backspace]

Change Summary

What and Why:

This changes fly domains disable to fly domains add, which appears to be what the subcommand actually invokes. I noticed the discrepancy in this output:

❯ fly domains
Command "domains" is deprecated, `fly domains` will be removed in a future release
Manage domains Notice: this feature is deprecated and no longer supported.
You can still view existing domains, but registration is no longer
possible.

Usage:
  flyctl domains [command]

Available Commands:
  disable     Add a domain
  list        List domains
  show        Show domain

How:

Since the function is newDomainsAdd and the function it calls is runDomainsCreate, this changes the description to match.

Related to:

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Documentation

• [ ] Fresh Produce
• [ ] In superfly/docs, or asked for help from docs team
• [ ] n/a