Funny, there are no classes named "ECS" in that link. In fact, there are no classes at all, because you didn't actually link a class source. Why push the step of searching for the class towards the reader when you could do it yourself? Wasn't it easy? I wonder why?
Taking a cursory look (which is already beyond my responsibilities) at https://github.com/redxdev/ECS/blob/master/ECS.h, by searching "class ECS" and "struct ECS" and "EntityComponentSystem" wields no results.