docs: remove language specifier from lesson code fence

- Changed from ```markdown to just ```
- Allows markdown formatting (like **bold**) to render inside the code block
- Title now displays as bold as it should

Author: UltimaPhoenix

docs/index.mdx

@@ -18,7 +18,7 @@ You're shipping more than ever and understanding less. devcoach fixes that with
 
 You work on a task with your AI agent as usual. When it finishes, devcoach appends a lesson:
 
-```markdown
+```
 🎓 devcoach · TypeScript · Level: Mid
 
 **Promise.allSettled vs Promise.all — don't let one failure sink the batch**