[editorial] 14.2 Usage guide on Dataset Series

[jimjyang]

Since the class Dataset Series in DCAT-AP now is not a subclass of dcat:Dataset but dcat:Resource, I think several of the bullet points in 14.2 Usage guide on Dataset Series need to be rewritten, especially the 3rd one about distributions (thus also the last sentence in the 4th one), the 5th one about data services, some parts of the 6th one (Dataset series being a member of Dataset series), the last one about metadata quality.