Fix capitalization of GitHub in User Guide
Fix capitalization of GitHub in User Guide

kotfic/girder,sutartmelson/girder,RafaelPalomar/girder,girder/girder,girder/girder,Kitware/girder,manthey/girder,adsorensen/girder,adsorensen/girder,adsorensen/girder,Xarthisius/girder,RafaelPalomar/girder,Xarthisius/girder,kotfic/girder,Kitware/girder,RafaelPalomar/girder,RafaelPalomar/girder,kotfic/girder,Kitware/girder,Xarthisius/girder,jbeezley/girder,kotfic/girder,sutartmelson/girder,jbeezley/girder,manthey/girder,kotfic/girder,manthey/girder,Kitware/girder,sutartmelson/girder,data-exp-lab/girder,data-exp-lab/girder,sutartmelson/girder,girder/girder,RafaelPalomar/girder,data-exp-lab/girder,sutartmelson/girder,Xarthisius/girder,manthey/girder,jbeezley/girder,data-exp-lab/girder,data-exp-lab/girder,adsorensen/girder,girder/girder,adsorensen/girder,Xarthisius/girder,jbeezley/girder