docs: fix typo (#1426)

docs/guide/theme-introduction.md

@@ -65,7 +65,7 @@ export default {
     // app is the Vue 3 app instance from `createApp()`.
     // router is VitePress' custom router. `siteData` is
     // a `ref` of current site-level metadata.
-  }
+  },
 
   setup() {
     // this function will be executed inside VitePressApp's