Wrong variable naming in code example (#3983) (#3988)

Co-authored-by: Sebastian Schuchmann <schuchmannsebastian@gmail.com>

docs/Using-Tensorboard.md

 
 ```csharp
 var statsRecorder = Academy.Instance.StatsRecorder;
-statsSideChannel.Add("MyMetric", 1.0);
+statsRecorder.Add("MyMetric", 1.0);
 ```