move Startup (#3357)

Project/Assets/ML-Agents/Examples/Startup/Scripts/Startup.cs

 using UnityEngine;
 using UnityEngine.SceneManagement;
 
-namespace MLAgents
+namespace MLAgentsExamples
 {
     internal class Startup : MonoBehaviour
     {

Project/Assets/ML-Agents/Examples/Startup/Scripts.meta

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