Eqn(6) in RDG doco fixed
Eqn(6) in RDG doco fixed

Fixes #12233

permcody/moose,jessecarterMOOSE/moose,idaholab/moose,nuclear-wizard/moose,dschwen/moose,dschwen/moose,sapitts/moose,permcody/moose,idaholab/moose,bwspenc/moose,jessecarterMOOSE/moose,lindsayad/moose,laagesen/moose,jessecarterMOOSE/moose,andrsd/moose,YaqiWang/moose,harterj/moose,nuclear-wizard/moose,andrsd/moose,nuclear-wizard/moose,harterj/moose,harterj/moose,lindsayad/moose,laagesen/moose,sapitts/moose,YaqiWang/moose,SudiptaBiswas/moose,bwspenc/moose,bwspenc/moose,nuclear-wizard/moose,bwspenc/moose,laagesen/moose,milljm/moose,permcody/moose,milljm/moose,sapitts/moose,SudiptaBiswas/moose,andrsd/moose,sapitts/moose,bwspenc/moose,SudiptaBiswas/moose,jessecarterMOOSE/moose,andrsd/moose,dschwen/moose,idaholab/moose,lindsayad/moose,idaholab/moose,lindsayad/moose,harterj/moose,jessecarterMOOSE/moose,lindsayad/moose,YaqiWang/moose,dschwen/moose,YaqiWang/moose,laagesen/moose,sapitts/moose,idaholab/moose,harterj/moose,milljm/moose,laagesen/moose,SudiptaBiswas/moose,dschwen/moose,milljm/moose,andrsd/moose,permcody/moose,milljm/moose,SudiptaBiswas/moose