01-responsive-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/advanced-grid/01-responsive-holy-grail/README.md

@@ -17,12 +17,12 @@ When the browser is stretched wide:
 ![desired outcome wide](./desired-outcome-wide.png)
 
 ### Self Check
-- The gap is 15px
-- The grid has two columns
-- The grid has four rows
-- The grid tracks do not use static sizes (no pixels!)
-- The second column is three times larger than the other
-- The third row is five times larger than the others
-- The columns narrow and widen as the viewport changes size
-- Row 3 gets taller to accomodate text wrapping as you narrow the viewport size 
+- [ ] The gap is 15px
+- [ ] The grid has two columns
+- [ ] The grid has four rows
+- [ ] The grid tracks do not use static sizes (no pixels!)
+- [ ] The second column is three times larger than the other
+- [ ] The third row is five times larger than the others
+- [ ] The columns narrow and widen as the viewport changes size
+- [ ] Row 3 gets taller to accomodate text wrapping as you narrow the viewport size