docs: update scripts:registry hook example

[bernhardberger]

docs: update script:registry hook example

🔗 Linked issue

Resolves #267

❓ 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)
• ✨ New feature (a non-breaking change that adds functionality)
• 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

The example in the docs on how to extend registry scripts via scripts:registry is not working. I updated it with a working example.

see #267

[vercel]

@bernhardberger is attempting to deploy a commit to the Nuxt Team on Vercel.

A member of the Team first needs to authorize it.

[harlan-zw]

Thank you!