docs(guide): remove duplicate backticks

remove duplicate backticks in code sample of theming

docs/guide/theming.md

@@ -42,7 +42,7 @@ export default {
   NotFound: () => 'custom 404', // <- this is a Vue 3 functional component
   enhanceApp({ app, router, siteData }) {
     // app is the Vue 3 app instance from `createApp()`. router is VitePress'
-    // custom router. `siteData`` is a `ref`` of current site-level metadata.
+    // custom router. `siteData` is a `ref` of current site-level metadata.
   }
 }
 ```