Updated CONTRIBUTING.md

Hi, I solved issue ows-ali#361, I changed the **underscore** symbol to **hyphen**. So, the problem might not come again. :)

CONTRIBUTING.md

@@ -7,7 +7,7 @@
 - Paste the solution code in new file and name it in following format:
 PROBLEM_NAME.[Format of language used in your code]
 (you should match it with the url, such that when a user type the url 
-hackkerrank.com/challenges/PROBLEM_NAME/problem 
+hackkerrank.com/challenges/PROBLEM-NAME/problem 
 codechef.com/problems/PROBLEM_NAME
 and so on
 it will be redirected to the actual problem page)