overviewControls: Fix thinko in SlideLayout

Controls that slide left are located on the left, so the offset to
align them with the corresponding edge is always 0. It's controls
on the right that need a different offset when the available width
exceeds the child's width.

https://bugzilla.gnome.org/show_bug.cgi?id=728899

js/ui/overviewControls.js

@@ -64,7 +64,7 @@ const SlideLayout = new Lang.Class({
         // flags only determine what to do if the allocated box is bigger
         // than the actor's box.
         let realDirection = getRtlSlideDirection(this._direction, child);
-        let alignX = (realDirection == SlideDirection.LEFT) ? (availWidth - natWidth) : 0;
+        let alignX = (realDirection == SlideDirection.RIGHT) ? (availWidth - natWidth) : 0;
 
         let actorBox = new Clutter.ActorBox();
         actorBox.x1 = box.x1 + alignX + this._translationX;