Fix bitmask.rst
Fix bitmask.rst

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