updated size on grid material

Project/Assets/ML-Agents/Examples/SharedAssets/Materials/GridMatFloor.mat

 Material:
   serializedVersion: 6
   m_ObjectHideFlags: 0
-  m_PrefabParentObject: {fileID: 0}
-  m_PrefabInternal: {fileID: 0}
+  m_CorrespondingSourceObject: {fileID: 0}
+  m_PrefabInstance: {fileID: 0}
+  m_PrefabAsset: {fileID: 0}
   m_Name: GridMatFloor
   m_Shader: {fileID: 4800000, guid: bbecf177c38d84ff08d7c683c116affd, type: 3}
   m_ShaderKeywords: 
     - _GlossMapScale: 1
     - _Glossiness: 0
     - _GlossyReflections: 1
-    - _GridSize: 6
+    - _GridSize: 24
     - _LineOffset: 0.5
     - _LineSize: 0.02
     - _Metallic: 0