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

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

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Tests

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