Update config.json

tests/validation/config.json

@@ -4,7 +4,7 @@
     {
       "title": "GUI StackPanel",
       "playgroundId": "#LLVZ90#0",
-      "referenceImage": "stackpanel.png"
+      "referenceImage": "StackPanel.png"
     },
     {
       "title": "Camera rig",
@@ -593,4 +593,4 @@
       "referenceImage": "outline.png"
     }
   ]
-}
+}