arrow

GH-38599: [Docs] Update Headers (#38696)

### Rationale for this change

Noticed wrong section headings on the web doc and proposing a fix.

### Are these changes tested?

Yes. Built and verified the documentation locally.

<img width="1049" alt="image" src="https://github.com/apache/arrow/assets/6668548/793d6222-2bc5-4f55-ab67-5abe5283add4">

### Are there any user-facing changes?

No.
* Closes: #38599

Authored-by: Hyunseok Seo <hsseo0501@gmail.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>