Fix broken link in CONTRIBUTING.md. (#353)

CONTRIBUTING.md

@@ -2,7 +2,7 @@
 
 This document contains information specific to contributing to the
 Project Pythia Portal. Please first refer to [Pythia Contributor's
-Guide](https://projectpythia.org/pages/contributing.html) for overall
+Guide](https://projectpythia.org/contributing.html) for overall
 contribution guidelines (such as detailed description of Project
 Pythia structure, forking, repository cloning, branching, etc.).