docs(examples): add key to dynamic asyncData

[huang-julien]

🔗 Linked issue

❓ 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

Async data keys should always uniquely identify a payload. Auto-keyed asyncData should be used in case of a unique payload. Data between dynamic routes are not considered as a unique payload since queryCollection('blog').path(route.path).first() returns a data depending on the current route.

📝 Checklist

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

[pkg-pr-new]

npm i https://pkg.pr.new/@nuxt/content@3332

commit: a478ae8