Add algorithms course by Harvard (#4599)

* add algorithms course

* update css50 course description

* update alphabetical order
Co-authored-by: NLarissa <larissa.moura@codeminer42.com>

free-courses-en.md

@@ -79,6 +79,7 @@
 * [Princeton University Algorithms, Part 2](https://www.coursera.org/learn/algorithms-part2)
 * [Stanford University Algorithms: Design and Analysis, Part 1](http://online.stanford.edu/course/algorithms-design-and-analysis-part-1)
 * [Stanford University Algorithms: Design and Analysis, Part 2](http://online.stanford.edu/course/algorithms-design-and-analysis-part-2)
+* [This is CS50x](https://cs50.harvard.edu/x/2020/) - David J. Malan (Harvard OpenCourseWare)
 
 
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