05-flex-modal: Add checkboxes to the README

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

Author: gs-101

foundations/flex/05-flex-modal/README.md

@@ -10,9 +10,9 @@ Depending on how you approach this one, you might need to revisit the `flex-shri
 
 ### Self Check
 
-- The blue icon is aligned to the left.
-- There is equal space on either side of the icon (the gaps between the icon and the edge of the card, and the icon and the text, are the same).
-- There is padding around the edge of the modal.
-- The header, text, and buttons are aligned with each other.
-- The header is bold and a slightly larger text-size than the text.
-- The close button is vertically aligned with the header, and aligned in the top-right of the card.
+- [ ] The blue icon is aligned to the left.
+- [ ] There is equal space on either side of the icon (the gaps between the icon and the edge of the card, and the icon and the text, are the same).
+- [ ] There is padding around the edge of the modal.
+- [ ] The header, text, and buttons are aligned with each other.
+- [ ] The header is bold and a slightly larger text-size than the text.
+- [ ] The close button is vertically aligned with the header, and aligned in the top-right of the card.