fix: replace broken flexbox-vs-grid link with MDN reference

The Web Dev Simplified blog post returned 404. Replaced it with the
MDN guide on the relationship between Grid and other layout methods,
which covers the same educational content.

Closes microsoft/Web-Dev-For-Beginners#1791

3-terrarium/2-intro-to-css/assignment.md

@@ -98,7 +98,7 @@ Understanding when and how to use different layout methods is a crucial skill fo
 ### Layout Method Guides
 - 📖 [A Complete Guide to Flexbox](https://css-tricks.com/snippets/css/a-guide-to-flexbox/)
 - 📖 [A Complete Guide to CSS Grid](https://css-tricks.com/snippets/css/complete-guide-grid/)
-- 📖 [Flexbox vs Grid - Choose the Right Tool](https://blog.webdevsimplified.com/2022-11/flexbox-vs-grid/)
+- 📖 [Flexbox vs Grid - Choose the Right Tool](https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_grid_layout/Relationship_of_grid_layout_with_other_layout_methods)
 
 ### Browser Testing Tools
 - 🛠️ [Browser DevTools Responsive Mode](https://developer.chrome.com/docs/devtools/device-mode/)