fix(docs): remark and rehype plugin syntax

[toniengelhardt]

🔗 Linked issue

Resolves #242

❓ 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

Fixes syntax for remark and rehype plugins in README, which currently doesn't match the implementation.

[pkg-pr-new]

npm i https://pkg.pr.new/@nuxtjs/mdc@416

commit: 827836c

[farnabaz]

Thanks