fix broken link in Problem Sets and Competitive Programming (#7384)

* fix broken link

* Update problem-sets-competitive-programming.md

Author: paulaxisabel

more/problem-sets-competitive-programming.md

@@ -10,7 +10,7 @@
 ### Competitive Programming
 
 * [4Clojure](http://www.4clojure.com)
-* [A2 Online Judge](https://a2oj.com)
+* [A2 Online Judge](https://a2oj.netlify.app)
 * [Algorithms for Competitive Programming](https://cp-algorithms.com)
 * [APL Problem Solving Competition](https://contest.dyalog.com)
 * [AtCoder](https://atcoder.jp)
@@ -82,7 +82,7 @@
 ### Problem Sets
 
 * [500 Data structures and algorithms interview questions and their solutions in C++](https://www.quora.com/q/techiedelight/500-Data-Structures-and-Algorithms-interview-questions-and-their-solutions)
-* [A2 Online Judge](https://a2oj.com/ps)
+* [A2 Online Judge](https://a2oj.netlify.app)
 * [Advent Of Code](http://adventofcode.com)
 * [AdventJS - 25 días de retos con JavaScript](https://adventjs.dev) - Miguel Ángel Durán «midudev» *(GitHub account requested, not required)*
 * [Anarchy Golf](http://golf.shinh.org)