docs: fix ash_domains config example in Get Started section

[arcanemachine]

I believe the correction in #676 was slightly off the mark. This little fix adjusts the syntax to the correct value (I think).

[zachdaniel]

AFAIK the syntax you're replacing works. Perhaps they both work?

config :ash_hq,
  ash_domains: [
    AshHq.Accounts,
    AshHq.Blog,
    AshHq.Docs,
    AshHq.Github,
    AshHq.MailingList,
    AshHq.Discord
  ]

This is what we do in ash_hq for example. Will close for now, but if I'm wrong LMK and we'll reopen :)

[arcanemachine]

Sorry, that's probably just confusion on my part regarding the config syntax. I thought it would write the data to a different section/key.

Note to self: Test both things manually first 😅

EDIT: Yep, both syntaxes work the same.