devdraw: use %R not Fn-F3 for retina toggle

R=rsc
http://codereview.appspot.com/6854093

src/cmd/devdraw/cocoa-srv.c

diff --git a/src/cmd/devdraw/cocoa-srv.c b/src/cmd/devdraw/cocoa-srv.c
index 810d315..197fd51 100644
--- a/src/cmd/devdraw/cocoa-srv.c
+++ b/src/cmd/devdraw/cocoa-srv.c
@@ -381,7 +381,7 @@
 		nk = 0;
 		return;
 	}
-	if(c == KF+3) {
+	if(c == Kcmd+'r') {
 		if(forcedpi)
 			forcedpi = 0;
 		else if(displaydpi >= 200)