Fix example in physical type docs
Fix example in physical type docs

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