Change default radius for rounded corners to 16

`16pt` seems to be the smallest window corner radius used ([screenshot](https://miro.medium.com/v2/1*qrN1Kg69XgS31FVfQrIloQ.jpeg)). Increasing the default radius to this improves visual consistency

Author: Joristdh

Source/RoundedCorners.spoon/init.lua

@@ -25,8 +25,8 @@ obj.allScreens = true
 
 --- RoundedCorners.radius
 --- Variable
---- Controls the radius of the rounded corners, in points. Defaults to 12
-obj.radius = 12
+--- Controls the radius of the rounded corners, in points. Defaults to 16
+obj.radius = 16
 
 
 --- RoundedCorners.fullscreenOffset