Fix thisChapterAudioLinks duplication in API documentation

[Copilot]

Issue

The API documentation in docs/reference/README.md had an error in the TypeScript interface definition for TranslationBookChapter. The property for next chapter audio links was incorrectly named thisChapterAudioLinks instead of nextChapterAudioLinks, creating a duplicate property name.

Changes

This PR fixes the documentation to match the actual TypeScript interface defined in packages/helloao-tools/generation/api.ts:

1. Corrected property name: Changed the duplicate thisChapterAudioLinks on line 367 to nextChapterAudioLinks
2. Added missing type annotation: Added | null to both nextChapterAudioLinks and previousChapterAudioLinks to accurately reflect that these properties can be null
3. Fixed incorrect comment: Corrected the comment for previousChapterAudioLinks from "Null if this is the last chapter" to "Null if this is the first chapter"

Before

nextChapterAudioLinks: TranslationBookChapterAudioLinks | null;
thisChapterAudioLinks: TranslationBookChapterAudioLinks;  // ❌ Wrong - duplicate name
previousChapterAudioLinks: TranslationBookChapterAudioLinks;  // ❌ Missing | null

After

nextChapterAudioLinks: TranslationBookChapterAudioLinks | null;  // ✅ Correct
previousChapterAudioLinks: TranslationBookChapterAudioLinks | null;  // ✅ Fixed

The documentation now correctly matches the actual API implementation and the JSON examples already present in the same file.

Fixes #[issue number]

Original prompt

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This section details on the original issue you should resolve

<issue_title>thisChapterAudioLinks repeated in README.md</issue_title>
<issue_description>thisChapterAudioLinks is repeated instead of using nextChapterAudioLinks in the README.md file

</issue_description>

Comments on the Issue (you are @copilot in this section)

Fixes #25

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