fix docstring warning
fix docstring warning

pllim/astropy,MSeifert04/astropy,pllim/astropy,saimn/astropy,stargaser/astropy,larrybradley/astropy,aleksandr-bakanov/astropy,astropy/astropy,lpsinger/astropy,StuartLittlefair/astropy,StuartLittlefair/astropy,dhomeier/astropy,StuartLittlefair/astropy,aleksandr-bakanov/astropy,larrybradley/astropy,pllim/astropy,stargaser/astropy,saimn/astropy,stargaser/astropy,saimn/astropy,lpsinger/astropy,saimn/astropy,dhomeier/astropy,StuartLittlefair/astropy,dhomeier/astropy,astropy/astropy,mhvk/astropy,mhvk/astropy,MSeifert04/astropy,dhomeier/astropy,larrybradley/astropy,mhvk/astropy,aleksandr-bakanov/astropy,larrybradley/astropy,StuartLittlefair/astropy,astropy/astropy,lpsinger/astropy,mhvk/astropy,aleksandr-bakanov/astropy,astropy/astropy,astropy/astropy,lpsinger/astropy,stargaser/astropy,saimn/astropy,lpsinger/astropy,larrybradley/astropy,mhvk/astropy,pllim/astropy,MSeifert04/astropy,dhomeier/astropy,MSeifert04/astropy,pllim/astropy