docs: Use correct defaultItems in the React Slash Menu Example

[niclas-j]

import { BlockNoteView, useBlockNote, defaultReactSlashMenuItems } from "@blocknote/react";
import "@blocknote/core/style.css";

function App() {
  const newSlashMenuItems: ReactSlashMenuItem[] = defaultReactSlashMenuItems;

  // Edit newSlashMenuItems
  ...

  const editor = useBlockNote({ slashMenuItems: newSlashMenuItems });

  return <BlockNoteView editor={editor} />;
}

Currently the example says:
const newSlashMenuItems: ReactSlashMenuItem[] = defaultSlashMenuItems;
leading to a type mismatch

 type 'BaseSlashMenuItem[]' is not assignable to type 'ReactSlashMenuItem[]'.
  Type 'BaseSlashMenuItem' is missing the following properties from type 'ReactSlashMenuItem': group, iconts(2322)

[vercel]

The latest updates on your projects. Learn more about Vercel for Git ↗︎

Name	Status	Preview	Comments	Updated (UTC)
blocknote	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	May 8, 2023 0:36am
blocknote-website	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	May 8, 2023 0:36am

[matthewlipski]

Good catch!