docs: fix Express.js 4.x API documentation link

[Jerry-CodeHub]

Summary

• Fix incorrect Express.js documentation link in mock guide
• The link https://expressjs.com/en/api.html now redirects to Express 5.x docs
• Updated to correct 4.x URL: https://expressjs.com/en/4x/api.html

Changes

• docs/docs/docs/guides/mock.md - Chinese documentation
• docs/docs/docs/guides/mock.en-US.md - English documentation

Test plan

• Verified that https://expressjs.com/en/api.html shows Express 5.x API Reference
• Verified that https://expressjs.com/en/4x/api.html shows Express 4.x API Reference

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