docs: update nested router example

[FenLiaoYu]

🔗 Linked issue

nuxt/nuxt#13696

❓ Type of change

• 📖 Documentation (updates to the documentation or readme)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality like performance)
• ✨ New feature (a non-breaking change that adds functionality)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

Use eventHandler() to wrap the route handler

In the latest release of h3, we should use eventHandler to wrap our handlers.
https://github.com/unjs/h3/releases/tag/v0.8.0

Fix error: Invalid lazy handler result. It should be a function

We should return a handler function, not a Router, use router.handler or useBase can fix it.
In this case, I think it should be useBase('/api', router.handler)

📝 Checklist

• I have linked an issue or discussion.
• I have updated the documentation accordingly.

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[danielroe]

Thanks!