src/kcmkwin: fix botched indentation code for final checkbox

This line had several problems:
1. Using un-namespaced `units` properties
2. Using `width` in a Layout
3. Using largeSpacing instead of gridUnit for indentation

All are fixed with ths commit.

src/kcmkwin/kwindesktop/package/contents/ui/main.qml

@@ -305,7 +305,7 @@ KCM.ScrollViewKCM {
                 Layout.fillWidth: true
 
                 Item {
-                    width: units.largeSpacing
+                    Layout.preferredWidth: Kirigami.Units.gridUnit
                 }
 
                 QQC2.CheckBox {