update readme thumbnail
update readme thumbnail

projectbid7/map-tour-storytelling-template-js,regan-sarwas/map-tour-storytelling-template-js,regan-sarwas/map-tour-storytelling-template-js,jfitzgerald79/map-tour-storytelling-template-js,Esri/map-tour-storytelling-template-js,kcompher/map-tour-storytelling-template-js,kcompher/map-tour-storytelling-template-js,Esri/map-tour-storytelling-template-js,jfitzgerald79/map-tour-storytelling-template-js,projectbid7/map-tour-storytelling-template-js