Fix: Comment out `howtoflutter.dev` (site down) and update tag from `… (#6931)

* Fix: Comment out `howtoflutter.dev` (site down) and update tag from `@article@Variables` to `@official@Variables`

* Removed `Comment`

src/data/roadmaps/flutter/content/100-dart-basics/101-variables.md

@@ -9,5 +9,4 @@ Variables in Flutter can store values of different data types, such as numbers,
 
 Visit the following resources to learn more:
 
-- [@article@Variables](https://dart.dev/guides/language/language-tour#variables)
-- [@article@Dart - Variables](https://howtoflutter.dev/dart/variables/)
+- [@official@Variables](https://dart.dev/guides/language/language-tour#variables)