Updated installation guid to take care of missed geant4 reference in header.
Updated installation guid to take care of missed geant4 reference in header.

mastbaum/rat-pac,BenLand100/rat-pac,BenLand100/rat-pac,BenLand100/rat-pac,BenLand100/rat-pac,mastbaum/rat-pac,mastbaum/rat-pac,mastbaum/rat-pac,mastbaum/rat-pac,BenLand100/rat-pac