Fixed d4_vertical_active_link_neighbors doctest.
Fixed d4_vertical_active_link_neighbors doctest.

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