docs(examples): fix runtimeConfig usage in config-extends example

[blinpete]

❓ 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

TheruntimeConfig part of the example doesn't work.
Deprecated publicRuntimeConfig prop isn't supported since v3.0.0-rc.14 (due to #9029).

Changing publicRuntimeConfig to runtimeConfig.public in nuxt.config.ts.

📝 Checklist

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

[codesandbox]

Open in CodeSandbox Web Editor | VS Code | VS Code Insiders