fix(v6): use consistent headings level in astro:assets

[ArmandPhilippot]

Description (required)

Updates modules/astro-assets.mdx to fix inconsistent headings level under <Image />.

Because those headings were moved before the refactoring on main, it seems we (I) forgot to update some heading levels while merging the branch in v6.

Related issues & labels (optional)

• Suggested label: consistency/formatting, v6

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Tracked Files

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en/reference/modules/astro-assets.mdx	Source changed, localizations will be marked as outdated.

Warnings reference

Icon	Description
🔄️	The source for this localization has been updated since the creation of this pull request, make sure all changes in the source have been applied.

[sarah11918]

Thank you!