Fix heading level for Area Chart section

Co-authored-by: hotlong <50353452+hotlong@users.noreply.github.com>

Author: Copilot

docs/plugins/plugin-charts.mdx

@@ -214,7 +214,7 @@ const schema = {
 }
 ```
 
-#### Area Chart
+###### Area Chart
 
 ```tsx
 const schema = {