update raycast tags

Project/Assets/ML-Agents/Examples/PushBlock/Prefabs/PushBlockCollabArea.prefab

   - goal
   - blockLarge
   - blockSmall
+  - agent
   m_RaysPerDirection: 3
   m_MaxRayDegrees: 90
   m_SphereCastRadius: 0.5
   - goal
   - blockLarge
   - blockSmall
+  - agent
   m_RaysPerDirection: 3
   m_MaxRayDegrees: 90
   m_SphereCastRadius: 0.5
   - goal
   - blockLarge
   - blockSmall
+  - agent
   m_RaysPerDirection: 3
   m_MaxRayDegrees: 90
   m_SphereCastRadius: 0.5
   - goal
   - blockLarge
   - blockSmall
+  - agent
   m_RaysPerDirection: 3
   m_MaxRayDegrees: 90
   m_SphereCastRadius: 0.5