chore: update doctl plugin reference

[bstoutenburgh]

Summary

Description:

• Tool repo URL: https://github.com/digitalocean/doctl/
• Plugin repo URL: https://github.com/bstoutenburgh/asdf-doctl

Checklist

• Format with scripts/format.bash
• Test locally with scripts/test_plugin.bash --file plugins/<your_new_plugin_name>
• Your plugin CI tests are green
  • Tip: use the plugin_test action from asdf-actions in your plugin CI

This is an older plugin. I had changed my github username, this updates the reference before redirects stop working.

$ curl -s -w '%{http_code} - %{redirect_url}\n' "https://github.com/maristgeek/asdf-doctl/"
301 - https://github.com/bstoutenburgh/asdf-doctl

[jthegedus]

Thanks