[docs] Fix heading formatting in prestissimo/prestissimo-features.rst

[steveburnett]

Description

Fix formatting of headings in prestissimo/prestissimo-features.rst.

Motivation and Context

Reduce warnings in the docs build process, as reported in PR 22311.

Impact

Documentation.

Test Plan

Local build, verified the page built cleanly.

Contributor checklist

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• PR description addresses the issue accurately and concisely. If the change is non-trivial, a GitHub Issue is referenced.
• Documented new properties (with its default value), SQL syntax, functions, or other functionality.
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• Adequate tests were added if applicable.
• CI passed.

Release Notes

== NO RELEASE NOTE ==