Updated the problem-sets-competitive-programming.md file (#7956)

* Updated the problem-sets-competitive-programming.md file

Added one Most useful DSA Learning problems set i.e., DSA Learning Series problems set (Codechef)

* Reordered in alphabetical order

* Updated the file as per the instructions

more/problem-sets-competitive-programming.md

@@ -101,6 +101,7 @@
 * [Coding Bat](http://codingbat.com/java)
 * [Coding Interview Questions and answers for practice \| Python, Java & C++](https://www.codingninjas.com/codestudio/problems) - CodingNinjas
 * [CSES Problem Set](https://cses.fi/problemset/)
+* [DSA Learning Series](https://www.codechef.com/LEARNDSA)
 * [Dynamic A2OJ Ladder](https://a2oj.herokuapp.com)
 * [Edabit](https://edabit.com)
 * [Exercism](http://exercism.io)