increase gravity to 1.5

Project/Assets/ML-Agents/Examples/Walker/Scenes/WalkerStatic.unity

   m_Script: {fileID: 11500000, guid: ec51f47c5ed0478080c449c74fd9c154, type: 3}
   m_Name: 
   m_EditorClassIdentifier: 
-  gravityMultiplier: 1
+  gravityMultiplier: 1.5
   fixedDeltaTime: 0.01333
   maximumDeltaTime: 0.15
   solverIterations: 12