Converted GitHub hyperlinks to relative links

Shadrock-  I just learned about relative links, and thought you might want to start using them- https://help.github.com/articles/relative-links-in-readmes.  I liked the way that your output heat map appears embedded in the README page, and have used the '!' tag myself- https://github.com/RRLash/Git-It-Done/blob/master/Tutorials/Custom-Color-Ramp-ArcMap/Custom-Color-Ramp-ArcMap.md