"Authenticating Git with GitHub: browser-based sign-in (OAuth)" should be a heading

In `docs/00-pre-workshop-setup.md`, the text "Authenticating Git with GitHub: browser-based sign-in (OAuth)" on line 148 is plain text but should be a heading (for example, `### Authenticating Git with GitHub: browser-based sign-in (OAuth)`) to improve document structure and screen reader navigation.