docs: update ContentRenderer custom components

[x] 📖 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)

Updated the docs for ContentRenderer as it was misleading. When using custom components in a MD file, user should pass a map which should be

const components = {
  'custom-component': CustomComponentObject,
}

Hope that help

nuxt / content