Use correct region name

Author: AristurtleDev

articles/tutorials/building_2d_games/21_customizing_gum_ui/snippets/optionsslider.cs

@@ -41,7 +41,7 @@ public OptionsSlider(TextureAtlas atlas)
         topLevelContainer.Height = 55f;
         topLevelContainer.Width = 264f;
 
-        TextureRegion backgroundRegion = atlas.GetRegion("slider-background");
+        TextureRegion backgroundRegion = atlas.GetRegion("panel-background");
 
         // Create the background panel that contains everything
         NineSliceRuntime background = new NineSliceRuntime();