Add CTF resource, check_urls=problem-sets-competitive-programming.md (#5133)

problem-sets-competitive-programming.md

@@ -48,6 +48,7 @@
 
 * [CTFlearn](https://ctflearn.com) (email adress *requested*)
 * [Google Ctf](https://capturetheflag.withgoogle.com) (email adress *requested*)
+* [Hacker101](https://ctf.hacker101.com) (email adress *requested*)
 * [Hackthebox](https://www.hackthebox.eu) (email adress *requested*)
 * [Overthewire Wargames fungame to practice CTF](https://overthewire.org/wargames/bandit)
 * [Picoctf](https://picoctf.org/resources) (email adress *requested*)