Update free-courses-en.md (#4699)

free-courses-en.md

@@ -130,7 +130,7 @@
 
 * [C Programming Tutorial](https://www.youtube.com/playlist?list=PL_RGaFnxSHWoGzOXqtKeM71OLpvZbuU0P)
 * [C Programming Tutorial for Beginners](https://www.youtube.com/watch?v=KJgsSFOSQv0) - Mike Dane - (freeCodeCamp)
-* [Introduction to Computer Science](https://www.edx.org/course/introduction-computer-science-harvardx-cs50x#.VNsjDvmUcmQ)
+* [Introduction to Computer Science](https://www.edx.org/course/cs50s-introduction-to-computer-science)
 * [The Arduino Platform and C Programming](https://www.coursera.org/learn/arduino-platform)