docs: Typo in Nuxt internal route check + add nuxt 4 warning

[maximepvrt]

Turns out the build path (buildAssetsDir) wasn’t correct https://nuxt.com/docs/3.x/api/nuxt-config#buildassetsdir

🔗 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

📝 Checklist

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