Correct the CS50 alt URL

https://cs50.harvard.edu/ just redirects to Edx. The course is available at https://cs50.harvard.edu/x/

extras/courses.md

@@ -9,7 +9,7 @@ or that there was an alternative that fit better into the curriculum.
 Courses | Duration | Effort
 :-- | :--: | :--:
 [Introduction to Computational Thinking and Data Science](https://www.edx.org/course/introduction-computational-thinking-data-mitx-6-00-2x-2#!)([alt](https://ocw.mit.edu/courses/electrical-engineering-and-computer-science/6-0002-introduction-to-computational-thinking-and-data-science-fall-2016/))| 10 weeks | 15 hours/week
-[Introduction to Computer Science - CS50](https://www.edx.org/course/introduction-computer-science-harvardx-cs50x#!) ([alt](https://cs50.harvard.edu/)) | 12 weeks | 10-20 hours/week
+[Introduction to Computer Science - CS50](https://www.edx.org/course/introduction-computer-science-harvardx-cs50x#!) ([alt](https://cs50.harvard.edu/x/)) | 12 weeks | 10-20 hours/week
 [Introduction to Computer Science (Udacity)](https://www.udacity.com/course/intro-to-computer-science--cs101)| 7 weeks | 10-20 hours/week
 [An Introduction to Interactive Programming in Python (Part 1)](https://www.coursera.org/learn/interactive-python-1)| 5 weeks | -
 [An Introduction to Interactive Programming in Python (Part 2)](https://www.coursera.org/learn/interactive-python-2)| - | -