FlxGroup: fix invalid code example
FlxGroup: fix invalid code example

kill() returns Void
flabbyfabby/flixel-docs,Doppp/flixel-docs,flabbyfabby/flixel-docs,flabbyfabby/flixel-docs,Gama11/flixel-docs,jcward/flixel-docs,Gama11/flixel-docs,Doppp/flixel-docs,jcward/flixel-docs