update demonstration meta files (#4367)

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Project/Assets/ML-Agents/Examples/Walker/Demos/ExpertWalkerDyVS.demo.meta

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Project/Assets/ML-Agents/Examples/Walker/Demos/ExpertWalkerStVS.demo.meta

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Project/Assets/ML-Agents/Examples/Walker/Demos/ExpertWalkerSta.demo.meta

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