corrected pytest 'tmpdir' reference [docs only]
corrected pytest 'tmpdir' reference [docs only]

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