fix: consistent headings format in api-reference.mdx

[ArmandPhilippot]

Same as #13315

For context: In a #13021 (comment), we decided we preferred Type.prop (or object.prop) as headings instead of prop both for readability and linking. We already use this format in some modules reference (e.g. envField) but most of the modules were using the old format (e.g. action).

Description (required)

Uses a consistent heading format in api-reference.mdx.

Note that we'll need to update the config file in core to fix a link. I'll check the other references before to avoid doing this multiple times.

Related issues & labels (optional)

• Suggested label: v6, consistency/formatting

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en/guides/sessions.mdx	Source changed, localizations will be marked as outdated.
en/reference/api-reference.mdx	Source changed, localizations will be marked as outdated.
en/reference/configuration-reference.mdx	Source changed, localizations will be marked as outdated.
en/reference/integrations-reference.mdx	Source changed, localizations will be marked as outdated.
ja/guides/sessions.mdx	Localization changed, will be marked as complete.
pl/guides/sessions.mdx	Localization changed, will be marked as complete.

Warnings reference

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🔄️	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]

This one looks great, too!

[ArmandPhilippot]

Thank you!

Just noting that I wait to check the other references before merging this in order to make a single PR in core if other files also affect the configuration reference! (this shouldn't take weeks 😄 )

[ArmandPhilippot]

I'll check the links in the core repo and make a PR, so this one is ready to go!