missed rename "keep_screen_on" for ios

platform/iphone/app_delegate.mm

@@ -243,7 +243,7 @@ static int frame_count = 0;
 	view_controller.view = glView;
 	window.rootViewController = view_controller;
 
-    _set_keep_screen_on(bool(GLOBAL_DEF("display/set_keep_screen_on",true)) ? YES : NO);
+	_set_keep_screen_on(bool(GLOBAL_DEF("display/keep_screen_on",true)) ? YES : NO);
 	glView.useCADisplayLink = bool(GLOBAL_DEF("display.iOS/use_cadisplaylink",true)) ? YES : NO;
 	printf("cadisaplylink: %d", glView.useCADisplayLink);
 	glView.animationInterval = 1.0 / kRenderingFrequency;