fix: Fix the issue where the chunk index might be duplicated

[jacob-lcs]

Description

This pull request includes a small change to the chunkDocument function in core/indexing/chunk/chunk.ts. The change simplifies the incrementation of the index variable by directly incrementing it within the object assignment. Fix the issue where the chunk index might be duplicated.

Checklist

• I've read the contributing guide
• The relevant docs, if any, have been updated or created
• The relevant tests, if any, have been updated or created

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Tests

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[jacob-lcs]

I have read the CLA Document and I hereby sign the CLA

[recurseml]

✨ No issues found! Your code is sparkling clean! ✨

[jacob-lcs]

@sestinj I've updated the code, all CI checks have passed. Please take another look when you have time. If everything looks good, it can be merged. :)