[skip ci] fix typo

ml-agents/mlagents/trainers/stats.py

     ) -> None:
         is_training = "Not Training."
         if "Is Training" in values:
-            stats_summary = stats_summary = values["Is Training"]
+            stats_summary = values["Is Training"]
             if stats_summary.mean > 0.0:
                 is_training = "Training."