engine/release-notes: fix header inconsistency in 28.2.1 section (#23511)

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## Description

This fixes the TOC
<img width="419" height="460" alt="image"
src="https://github.com/user-attachments/assets/fb51921d-8650-422d-82bb-6eb69f375345"
/>


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## Related issues or tickets

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## Reviews

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- [ ] Technical review
- [ ] Editorial review
- [ ] Product review

Signed-off-by: Paweł Gronowski <[email protected]>

Author: vvoland

content/manuals/engine/release-notes/28.md

@@ -324,7 +324,7 @@ For a full list of pull requests and changes in this release, refer to the relev
 
 {{< release-date date="2025-05-29" >}}
 
-## Packaging updates
+### Packaging updates
 
 - Fix packaging regression in [v28.2.0](https://github.com/moby/moby/releases/tag/v28.2.0) which broke creating the `docker` group/user on fresh installations. [docker-ce-packaging#1209](https://github.com/docker/docker-ce-packaging/issues/1209)