GitLab OmniAuth provider - update code snippet for changing GitLab URL

What does this MR do?

The GitLab OmniAuth provider can be configured to point to another self-managed instance instead of GitLab.com as per the gem https://github.com/linchus/omniauth-gitlab#standalone-usage - however this document makes no mention of that.

This MR updates the code snippet to hint at how to customize the GitLab URL so we at least have a record of this somewhere. A more saavy person would see this and realise that the URL can be changed, if they wish to use their own self managed instance for the GitLab OmniAuth provider instead of GitLab.com.

The problem I have now is that this document is really focused on GitLab.com and I wasn't sure if I should go down the path of rewriting the doc so it applies to both GitLab.com and self-managed instances. Let me know what you think.

Related issues

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