Fixed doctest for horizontal_fixed_link_ids.
Fixed doctest for horizontal_fixed_link_ids.

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