add cs50 introduction to computer science (#7449)

courses/free-courses-en.md

@@ -136,7 +136,6 @@
 * [Sorting Algorithms](https://www.youtube.com/playlist?list=PL2_aWCzGMAwKedT2KfDMB9YA5DgASZb3U) - mycodeschool
 * [Stanford University Algorithms: Design and Analysis, Part 1](https://online.stanford.edu/courses/soe-ycsalgorithms1-algorithms-design-and-analysis-part-1)
 * [Stanford University Algorithms: Design and Analysis, Part 2](https://online.stanford.edu/courses/soe-ycs0001-algorithms-design-and-analysis-part-2)
-* [This is CS50x](https://cs50.harvard.edu/x/2021) - David J. Malan (Harvard OpenCourseWare)
 
 
 ### Android
@@ -289,6 +288,7 @@
 ### Computer Science
 
 * [Berkeley's CS  61A: Taught using SICP](https://archive.org/details/ucberkeley-webcast-PL3E89002AA9B9879E?tab=collection)
+* [CS50's Introduction to Computer Science](https://cs50.harvard.edu/x/) - David J. Malan (Harvard OpenCourseWare and edX)
 * [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)
 * [MIT 6.824 Distributed Systems (Spring 2020)](https://www.youtube.com/playlist?list=PLrw6a1wE39_tb2fErI4-WkMbsvGQk9_UB)