Added free course on red black trees. (#7547)

* Added Red Black trees course

* Update free-courses-en.md

courses/free-courses-en.md

@@ -135,6 +135,7 @@
 * [Online Java + DSA + Interview preparation course](https://www.youtube.com/playlist?list=PL9gnSGHSqcnr_DxHsP7AW9ftq0AtAyYqJ) - Kunal Kushwaha
 * [Princeton University Algorithms, Part 1](https://www.coursera.org/learn/algorithms-part1)
 * [Princeton University Algorithms, Part 2](https://www.coursera.org/learn/algorithms-part2)
+* [Red Black Tree in Data Structures](https://youtube.com/playlist?list=PLPzfPcir5uPT8KFST1Ba3vN6k9yKE9ZK4) - NG Tutorials
 * [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)