increase drag

Project/Assets/ML-Agents/Examples/Tennis/Prefabs/TennisArea.prefab

   m_GameObject: {fileID: 1273406647218856}
   serializedVersion: 2
   m_Mass: 0.75
-  m_Drag: 0.1
+  m_Drag: 0.2
   m_AngularDrag: 0.05
   m_UseGravity: 1
   m_IsKinematic: 0