Fixed doctest for horizontal_active_link_ids.
Fixed doctest for horizontal_active_link_ids.

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