Update definition of Layer (Section 2.13)

With the expansion of the ArchiMate language over the years, the concept of layering was also expanded to encompass not only "An abstraction of the ArchiMate framework at which an enterprise can be modeled." to represent the many architectural domains ArchiMate can model. So, a new broader definition is proposed:

"An abstraction of the ArchiMate framework to classify elements according to their architectural domain."