Edited the Tennis code and retrained the model (#1746)

Addressing #1739

UnitySDK/Assets/ML-Agents/Examples/Tennis/Scripts/TennisAgent.cs

     public override void InitializeAgent()
     {
         agentRb = GetComponent<Rigidbody>();
-        ballRb = GetComponent<Rigidbody>();
+        ballRb = ball.GetComponent<Rigidbody>();
         var canvas = GameObject.Find(CanvasName);
         GameObject scoreBoard;
         if (invertX)

UnitySDK/Assets/ML-Agents/Examples/Tennis/TFModels/TennisLearning.nn.meta

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+guid: 0b22bc7e45e8347fd897bdf0e1d38c6e
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