Add TechGig, check_urls=problem-sets-competitive-programming.md (#5130)

problem-sets-competitive-programming.md

@@ -38,6 +38,7 @@
 * [Microcorruption](https://microcorruption.com/login)
 * [oj.uz](https://oj.uz)
 * [Sphere Online Judge](http://www.spoj.com/contests)
+* [Techgig](https://www.techgig.com)
 * [Topcoder](https://www.topcoder.com)
 * [Toph](https://toph.co)
 * [URI Online Judge](https://www.urionlinejudge.com.br/judge/en/login)