Update model_grid.rst
Update model_grid.rst

fixed math mistake
decvalts/landlab,Carralex/landlab,amandersillinois/landlab,cmshobe/landlab,amandersillinois/landlab,csherwood-usgs/landlab,SiccarPoint/landlab,cmshobe/landlab,laijingtao/landlab,RondaStrauch/landlab,ManuSchmi88/landlab,Carralex/landlab,landlab/landlab,cmshobe/landlab,RondaStrauch/landlab,RondaStrauch/landlab,Carralex/landlab,laijingtao/landlab,ManuSchmi88/landlab,ManuSchmi88/landlab,landlab/landlab,SiccarPoint/landlab,csherwood-usgs/landlab,landlab/landlab,decvalts/landlab