fix flatten comment
fix flatten comment

jridgewell/underscore,platinumazure/underscore,jashkenas/underscore,vincent1988/underscore,huangnan1/2016gitStudy,vincent1988/underscore,akre54/underscore,huangnan1/2016gitStudy,jpalomar/underscore,documentcloud/underscore,jashkenas/underscore,jpalomar/underscore,SeriaWei/underscore,rlugojr/underscore,SeriaWei/underscore,jridgewell/underscore,platinumazure/underscore,rlugojr/underscore,tnga/underscore-es,jdelbello/underscore,documentcloud/underscore,jdelbello/underscore,keepgoingwm/underscore,keepgoingwm/underscore,tnga/underscore-es,akre54/underscore