add problemset/grind75 to problem sets category (#9873)

* Added Grind 75 problem set

* minor fixes

more/problem-sets-competitive-programming.md

@@ -109,6 +109,7 @@
 * [Edabit](https://edabit.com)
 * [Exercism](http://exercism.io)
 * [Geeks For Geeks](https://practice.geeksforgeeks.org)
+* [Grind 75](https://www.techinterviewhandbook.org/grind75)
 * [Hacker.org](http://www.hacker.org)
 * [HackerEarth](https://www.hackerearth.com)
 * [HDU Online Judge](http://acm.hdu.edu.cn)
@@ -134,3 +135,4 @@
 * [UVa Online Judge](https://uva.onlinejudge.org/index.php?Itemid=8&option=com_onlinejudge)
 * [Vulnhub.com](https://www.vulnhub.com)
 * [Школа программиста](https://acmp.ru)
+