fix UI sample image
fix UI sample image

Microsoft/vsts-extension-samples,Microsoft/vsts-extension-samples,Microsoft/vso-extension-samples,srdjan-bozovic/vso-extension-samples,srdjan-bozovic/vso-extension-samples,srdjan-bozovic/vso-extension-samples,Microsoft/vsts-extension-samples,Microsoft/vso-extension-samples,Microsoft/vso-extension-samples