fix(code-block): pass language as class name

[stefanprobst]

🔗 Linked issue

❓ 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

some syntax highlighters and other packages which transform markdown codeblocks (e.g. mermaid) expect the language of the code block to be passed via class attribute. this fixed that.

for context: i ran into this issue trying to add rehype-mermaidjs to rehypePlugins.

📝 Checklist

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

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