Modified the href for contributing.md in index.html to point to github

Author: trevordblack

index.html

@@ -76,7 +76,7 @@ <h1 id='issues'>Issues</h1>
 
 
   <h1 id='contributing'>Contributing</h1>
-  <p>Interested in helping out? Please read the guidelines in the <a href="CONTRIBUTING.md">CONTRIBUTING.md</a> document
+  <p>Interested in helping out? Please read the guidelines in the <a href="https://github.com/RayTracing/raytracing.github.io/blob/master/CONTRIBUTING.md">CONTRIBUTING.md</a> document
   first. <em>Pull requests without associated issues, or submitted without coordination, are highly likely to be
   rejected.</em>