Editorial: Fix incorrect heading level in example

[MarcusOtter]

The heading levels in this example are not valid after the outline algorithm changes, because it contains two <h1> tags. This PR changes the 2nd <h1> to an <h2> which should be semantically correct in this example.

• This change is editorial
• The top of this comment includes a clear commit message to use.

---

/acknowledgements.html ( diff )
/sections.html ( diff )

[MarcusOtter]

Sidenote, the contributing guidelines state that:

Please add your name to the Acknowledgments section (search for <!-- ACKS) in your first pull request, even for trivial fixes.

I assumed this also includes trivial editorial fixes, happy to remove that line if not.

[annevk]

Thanks @MarcusOtter!