[QB] One more editorial fix

In answering a user question I noticed a small error in the Data Cube spec.

The domain of qb:observation was mis-stated in the reference section 
(was qb:Slice but with the introduction of the super class 
qb:ObservationGroup then the domain should be qb:ObservationGroup).

The domain was shown correctly in the diagram, the main text and the 
ontology itself but not in the reference section.

Given that this is a small editorial fix I have corrected this in the 
editors draft and updated the changes section, the static CR-draft and 
the diff.

Dave