Add Flexbox in 30 days tutorial (#3441)

free-courses-en.md

@@ -199,6 +199,7 @@
 * [Code Your First Game: Arcade Classic in JavaScript on Canvas](https://www.udemy.com/code-your-first-game/) - Chris DeLeon (Udemy)
 * [CSS Flexbox - Mastering the Basics](https://www.udemy.com/css-flexbox-mastering-the-basics/) - Vishwas Gopinath (Udemy)
 * [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)
 * [Learn how to program: CSS](https://www.learnhowtoprogram.com/css) - Epicodus Inc.
 * [Learn HTML5 Programming From Scratch](https://www.udemy.com/learn-html5-programming-from-scratch/)