increase grav multiplier (#4283)

Project/Assets/ML-Agents/Examples/OldCrawler/Prefabs/OldCrawler.prefab

       propertyPath: m_Name
       value: OldCrawler
       objectReference: {fileID: 0}
+    - target: {fileID: 6234525248087620104, guid: 0456c89e8c9c243d595b039fe7aa0bf9,
+        type: 3}
+      propertyPath: m_IsActive
+      value: 0
+      objectReference: {fileID: 0}
     m_RemovedComponents:
     - {fileID: 4845971001715176649, guid: 0456c89e8c9c243d595b039fe7aa0bf9, type: 3}
   m_SourcePrefab: {fileID: 100100000, guid: 0456c89e8c9c243d595b039fe7aa0bf9, type: 3}

Project/Assets/ML-Agents/Examples/OldCrawler/Prefabs/OldFixedPlatform.prefab

   m_Modification:
     m_TransformParent: {fileID: 4309919623019186}
     m_Modifications:
+    - target: {fileID: 2841639107773815270, guid: e24df050e78384b24b065a2b14effd14,
+        type: 3}
+      propertyPath: m_IsActive
+      value: 0
+      objectReference: {fileID: 0}
     - target: {fileID: 3448064811409790574, guid: e24df050e78384b24b065a2b14effd14,
         type: 3}
       propertyPath: target

Project/Assets/ML-Agents/Examples/OldCrawler/Scenes/OldCrawlerStaticTarget.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