Making sorter inference use 20 tiles instead of 8 by default (#5253)

* Making sorter inference use 20 tiles instead of 8 by default

* removing the changes to the prefab due to version upgrade

Project/Assets/ML-Agents/Examples/Sorter/Prefabs/Area.prefab

     maxStep: 0
   hasUpgradedFromAgentParameters: 1
   MaxStep: 5000
-  DefaultMaxNumTiles: 8
+  DefaultMaxNumTiles: 20
   NumberTilesList:
   - {fileID: 1917831257363183701}
   - {fileID: 7854509637219396240}