Update free-courses-en.md (#8386)

* Update free-courses-en.md

Added Discrete Mathematics course

* Update free-courses-en.md

Changed to My Lesson

courses/free-courses-en.md

@@ -345,6 +345,7 @@
 * [Berkeley's CS  61A: Taught using SICP](https://archive.org/details/ucberkeley-webcast-PL3E89002AA9B9879E?tab=collection)
 * [Computer Networking - Network Engineering](https://www.youtube.com/watch?v=qiQR5rTSshw) - Brian Ferrill (FreeCodeCamp)
 * [CS50's Introduction to Computer Science](https://cs50.harvard.edu/x/) - David J. Malan (Harvard OpenCourseWare and edX)
+* [Discrete Mathematics for Computer Science Specialization](https://www.youtube.com/playlist?list=PLtS8Ubq2bIlXO4qEM5BOsBy6xWQNVFu8l) - My Lesson
 * [Introduction to Augmented Reality and ARCore](https://www.coursera.org/learn/ar) - Daydream
 * [LouvainX Paradigms of Computer Programming – Abstraction and Concurrency](https://www.edx.org/course/paradigms-computer-programming-louvainx-louv1-2x-1#!)
 * [LouvainX Paradigms of Computer Programming – Fundamentals](https://www.edx.org/course/paradigms-computer-programming-louvainx-louv1-1x-1)