01-basic-holy-grail: Add checkboxes to the README

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

Author: gs-101

intermediate-html-css/positioning-grid/01-basic-holy-grail/README.md

@@ -14,11 +14,11 @@ Now it's time to practice the Grid tools we've learned and create a layout that
 
 ### Self Check
 
-- The gap is 15px
-- The grid has two columns
-- The grid has four rows
-- The second column is three times larger than the first
-- The third row is five times larger than the others
-- The header and footer elements span across both columns
-- The sidebar element only spans across the first column
-- The nav and article elements span across the second column
+- [ ] The gap is 15px
+- [ ] The grid has two columns
+- [ ] The grid has four rows
+- [ ] The second column is three times larger than the first
+- [ ] The third row is five times larger than the others
+- [ ] The header and footer elements span across both columns
+- [ ] The sidebar element only spans across the first column
+- [ ] The nav and article elements span across the second column