Core#Dereferencing example schema: remove second dimension of array

[notEthan]

I think this is probably what was intended - I don't see any reason for the arrays in this example to be nested. If I'm wrong and this is how the schema is supposed to be, the reference here to the schema containing the #item reference should be corrected from #/items to #/items/items.

Introduced 23ba42e1 - ping @awwright, not that I expect you to remember these lines six years later.