fix: remove duplicate Extmarks demo entry (#860)

## Summary

- remove the duplicated `Extmarks Demo` registration from the examples
menu

Author: AbigailJixiangyuyu

packages/core/src/examples/index.ts

@@ -196,12 +196,6 @@ const examples: Example[] = [
     run: linkDemo.run,
     destroy: linkDemo.destroy,
   },
-  {
-    name: "Extmarks Demo",
-    description: "Virtual extmarks - text ranges that cursor jumps over, like inline tags and links",
-    run: extmarksDemo.run,
-    destroy: extmarksDemo.destroy,
-  },
   {
     name: "Opacity Demo",
     description: "Box opacity and transparency effects with animated opacity transitions",