Update free-courses-en.md (#4378)

* Update free-courses-en.md

Added Flexbox & CSS Grid free video resources to the repo

* Update free-courses-en.md

* Update free-courses-en.md

Removed the trailing slashes from the root domain

free-courses-en.md

@@ -236,6 +236,7 @@
 * [Code Your First Game: Arcade Classic in JavaScript on Canvas](https://www.udemy.com/code-your-first-game/) - Chris DeLeon (Udemy)
 * [Complete HTML/CSS BootCamp](https://frontendmasters.com/bootcamp/) - FrontEnd Masters
 * [CSS Flexbox - Mastering the Basics](https://www.udemy.com/css-flexbox-mastering-the-basics/) - Vishwas Gopinath (Udemy)
+* [CSS Grid](https://cssgrid.io) - Wesbos
 * [CSS Tutorial](https://www.youtube.com/playlist?list=PL_RGaFnxSHWqMH9a9DY8LFKrJ5NJCFHHe)
 * [Flexbox in 30 Days](https://github.com/samanthaming/Flexbox30) - Samantha Ming
 * [HTML Tutorial](https://www.youtube.com/playlist?list=PL_RGaFnxSHWr_FB-hVEgEGUESZL1TOiJ6)
@@ -243,6 +244,7 @@
 * [Learn HTML5 Programming From Scratch](https://www.udemy.com/learn-html5-programming-from-scratch/)
 * [Learn to style HTML using CSS](https://developer.mozilla.org/en-US/docs/Learn/CSS)
 * [Structuring the web with HTML](https://developer.mozilla.org/en-US/docs/Learn/HTML)
+* [What the Flexbox](https://flexbox.io) - Wesbos
 
 
 ### iOS