Fix a typo in a comment in ContainerState::FindThebesLayerFor. No bug, r=typo

layout/base/FrameLayerBuilder.cpp

@@ -2249,7 +2249,7 @@ ContainerState::FindThebesLayerFor(nsDisplayItem* aItem,
     ThebesLayerData* data = mThebesLayerDataStack[i];
     // Give up if there is content drawn above (in z-order) this layer that
     // intersects aItem's visible region; aItem must be placed in a
-    // layer this layer.
+    // layer above this layer.
     if (data->DrawAboveRegionIntersects(aVisibleRect)) {
       ++i;
       break;