Update problem-sets-competitive-programming.md (#7789)

* Update problem-sets-competitive-programming.md

Added a a website link to competitive programming

* Update more/problem-sets-competitive-programming.md

Co-authored-by: Eric Hellman <eric@hellman.net>

more/problem-sets-competitive-programming.md

@@ -33,6 +33,7 @@
 * [Dimik](https://dimikoj.com)
 * [DMOJ](https://dmoj.ca)
 * [E-olymp](https://www.e-olymp.com/en/)
+* [Exercism](https://exercism.org)
 * [Facebook Hackercup](https://www.facebook.com/hackercup)
 * [Google Code Jam](https://codingcompetitions.withgoogle.com/codejam)
 * [Google Kickstart](https://codingcompetitions.withgoogle.com/kickstart)