Fix Walker bug with dirToTarget (#3309)

Project/Assets/ML-Agents/Examples/Walker/Scripts/WalkerAgent.cs

         // b. Rotation alignment with goal direction.
         // c. Encourage head height.
         // d. Discourage head movement.
+        m_DirToTarget = target.position - m_JdController.bodyPartsDict[hips].rb.position;
         AddReward(
             +0.03f * Vector3.Dot(m_DirToTarget.normalized, m_JdController.bodyPartsDict[hips].rb.velocity)
             + 0.01f * Vector3.Dot(m_DirToTarget.normalized, hips.forward)