Fix incorrect code

Author: thammarith

snippets/auth-next/index/auth_set_language_code.js

@@ -10,5 +10,5 @@ import { getAuth } from "firebase/auth";
 const auth = getAuth();
 auth.languageCode = 'it';
 // To apply the default browser preference instead of explicitly setting it.
-// firebase.auth().useDeviceLanguage();
-// [END auth_set_language_code_modular]
+// auth.useDeviceLanguage();
+// [END auth_set_language_code_modular]