Update free-courses-en.md (#9447)

The link to "MIT's Mathematics for Computer Science" has changed to https://ocw.mit.edu/courses/6-042j-mathematics-for-computer-science-fall-2010/

courses/free-courses-en.md

@@ -395,7 +395,7 @@
 * [MIT's Computer Language Engineering](http://ocw.mit.edu/courses/electrical-engineering-and-computer-science/6-035-computer-language-engineering-sma-5502-fall-2005/lecture-notes/)
 * [MIT's Introduction to Computer Science and Programming](https://ocw.mit.edu/courses/electrical-engineering-and-computer-science/6-00sc-introduction-to-computer-science-and-programming-spring-2011/) - John Guttag (MIT OpenCourseWare)
 * [MIT's Introduction to Computer Science and Programming in Python](https://ocw.mit.edu/courses/electrical-engineering-and-computer-science/6-0001-introduction-to-computer-science-and-programming-in-python-fall-2016/) - Ana Bell, Eric Grimson, John Guttag (MIT OpenCourseWare)
-* [MIT's Mathematics for Computer Science](http://ocw.mit.edu/courses/electrical-engineering-and-computer-science/6-042j-mathematics-for-computer-science-fall-2010/video-lectures/)
+* [MIT's Mathematics for Computer Science](https://ocw.mit.edu/courses/6-042j-mathematics-for-computer-science-fall-2010/)
 
 
 ### Cryptography