Update free-courses-en.md (#3333)

free-courses-en.md

@@ -194,6 +194,7 @@
 * [Build a responsive website with Webflow](https://www.bloc.io/tutorials/webflow-tutorial-design-responsive-sites-with-webflow)
 * [Build a SaaS landing page using Skeleton](https://www.bloc.io/tutorials/jottly-a-beginner-s-guide-to-html-css-skeleton-and-animate-css)
 * [Build Dynamic Websites](http://cs75.tv/2010/fall/)
+* [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)
 * [HTML Tutorial](https://www.youtube.com/playlist?list=PL_RGaFnxSHWr_FB-hVEgEGUESZL1TOiJ6)
 * [Learn how to program: CSS](https://www.learnhowtoprogram.com/css) - Epicodus Inc.