docs(README): fix link to MDC syntax

[Vinccool96]

The first link to MDC syntax was pointing to nothing. It is now pointing at https://content.nuxtjs.org/guide/writing/mdc

🔗 Linked issue

No issue linked

❓ 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.

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[nobkd]

Consider changing your PR title to follow the Conventional Commits Syntax as noted in the comment at the top of the PR template.
e.g. add a docs to the front: docs: Fix link to MDC syntax in README.md