docs: move fetchContentNavigation into function

[nobkd]

🔗 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

The documentation was missing that fetchContentNavigation() has to be in a function:

[nuxt] [request error] [nuxt] [asyncData] handler must be a function.
at createError (./node_modules/h3/dist/index.mjs:196:15) at Server.nodeHandler (./node_modules/h3/dist/index.mjs:386:21) at processTicksAndRejections (node:internal/process/task_queues:96:5)

This is also visible in the example

📝 Checklist

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

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