Differences between docs and `app.ini` for `SSH_BACKUP_AUTHORIZED_KEYS` setting

[pat-s]

The docs for getting SSH working when using the Docker installation report a different env var name than the one that is actually stored in app.ini.

• app.ini: SSH_AUTHORIZED_KEYS_BACKUP :

  gitea/custom/conf/app.example.ini

  Line 318 in 06eaf74

  SSH_AUTHORIZED_KEYS_BACKUP = true

  (link)
• Docs: SSH_BACKUP_AUTHORIZED_KEYS (link)

I assume the docs are out of date and will issue a PR.