docs: correct broken CONTRIBUTING.md link in apps/dev/nextjs-v4

[danlzh]

NOTE:

• It's a good idea to open an issue first to discuss potential changes.
• Please make sure that you are NOT opening a PR to fix a potential security vulnerability. Instead, please follow the Security guidelines to disclose the issue to us confidentially.

☕️ Reasoning

CONTRIBUTING.md has moved under the .github folder but the reference link in apps/dev/nextjs-v4/README.md is not updated.

Updated the document link in to point to the correct CONTRIBUTING.md

🧢 Checklist

• Documentation
• Tests
• Ready to be merged

🎫 Affected issues

Please scout and link issues that might be solved by this PR.

Fixes: INSERT_ISSUE_LINK_HERE

📌 Resources

• Security guidelines
• Contributing guidelines
• Code of conduct
• Contributing to Open Source

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