add wall tag

Project/Assets/ML-Agents/Examples/CubeWars/Prefabs/CubeWarArea.prefab

       propertyPath: m_IsActive
       value: 1
       objectReference: {fileID: 0}
+    - target: {fileID: 100000, guid: 8c40fb8b2e877464087ee4920444a812, type: 3}
+      propertyPath: m_TagString
+      value: wall
+      objectReference: {fileID: 0}
     - target: {fileID: 400000, guid: 8c40fb8b2e877464087ee4920444a812, type: 3}
       propertyPath: m_LocalPosition.x
       value: 0