[README] Fix example.

README.md

@@ -19,7 +19,7 @@ function love.load()
     local x = math.floor((love.graphics.getWidth() - w) / 2)
     local y = math.floor((love.graphics.getHeight() - h) / 2)
 
-    ui = yui.Ui.new {
+    ui = yui.Ui:new {
         x = x, y = y,
 
         yui.Rows {