fix setTeamReward

Co-authored-by: Vincent-Pierre BERGES <vincentpierre@unity3d.com>

com.unity.ml-agents/Runtime/Agent.cs

 #if DEBUG
             Utilities.DebugCheckNanAndInfinity(reward, nameof(reward), nameof(SetTeamReward));
 #endif
-            m_TeamReward += reward;
+            m_TeamReward = reward;
         }
 
         public void AddTeamReward(float increment)