03-dropdown-menu: Add checkboxes to the README

Add checkboxes to the README to make the progress tracking a better experience.

Author: gs-101

advanced-html-css/animation/03-dropdown-menu/README.md

@@ -14,5 +14,5 @@ Your task is to add animation to the dropdown menu so that it will have an effec
 
 ### Self Check
 
-- The dropdown menu expands after you click on the menu title
-- There's a _bounce_ illusion towards the end of the animation
+- [ ] The dropdown menu expands after you click on the menu title
+- [ ] There's a _bounce_ illusion towards the end of the animation