doc: unused array variable extension fixed in the config example (#672)

Author: alpha-og

Diff for: core/README.md

@@ -277,7 +277,7 @@ export default function App() {
       value="console.log('hello world!');"
       height="200px"
       theme={okaidia}
-      extensions={[javascript({ jsx: true })]}
+      extensions={extensions}
     />
   );
 }