added another doctest to divergence.py
added another doctest to divergence.py

laijingtao/landlab,cmshobe/landlab,amandersillinois/landlab,csherwood-usgs/landlab,Carralex/landlab,RondaStrauch/landlab,amandersillinois/landlab,cmshobe/landlab,landlab/landlab,landlab/landlab,RondaStrauch/landlab,laijingtao/landlab,Carralex/landlab,landlab/landlab,csherwood-usgs/landlab,RondaStrauch/landlab,ManuSchmi88/landlab,SiccarPoint/landlab,cmshobe/landlab,SiccarPoint/landlab,ManuSchmi88/landlab,Carralex/landlab,ManuSchmi88/landlab