docs: fix built-in wording in migration guide

[Rohan5commit]

Description

Replace "build-in" with "built-in" in the v4-to-v5 migration guide.

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Checklist

• Does npm test pass with this change (including linting)?
• Is the new or changed code fully tested?
• Is a documentation update included (if this change modifies existing APIs, or introduces new ones)?

Validation

• Ran git diff --check

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Note

Low Risk
Low risk documentation-only change that fixes a typo with no effect on runtime behavior.

Overview
Fixes a typo in the v4-to-v5 migration guide by correcting “build-in” to “built-in” in the Keep Alive section.

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[Copilot]

Pull request overview

This PR corrects a typo in the v4-to-v5 migration guide to use the proper “built-in” wording when referencing Node.js core modules.

Changes:

• Replace “build-in” with “built-in” in the Keep Alive section of the v4-to-v5 migration guide.

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