fix icon path (#3493)

com.unity.ml-agents/Editor/DemonstrationImporter.cs

     [ScriptedImporter(1, new[] {"demo"})]
     internal class DemonstrationImporter : ScriptedImporter
     {
-        const string k_IconPath = "Assets/ML-Agents/Resources/DemoIcon.png";
+        const string k_IconPath = "Packages/com.unity.ml-agents/Editor/Icons/DemoIcon.png";
 
         public override void OnImportAsset(AssetImportContext ctx)
         {