Fix restrictions on Offer to allow for offers to buy in addition to offers to sell

[rjyounes]

This restriction does not work for offers to buy:

			[
				a owl:Restriction ;
				owl:onProperty gist:offers ;
				owl:someValuesFrom gist:CatalogItem ;
			]

[stevenchalem]

Investopedia defines "Offer" as follows

An offer is a conditional proposal made by a buyer or seller to buy or sell an asset, which becomes legally binding if accepted. An offer is also defined as the act of offering something for sale, or the submission of a bid to buy something.

[rjyounes]

Yes. The text definition includes both offers to buy and offers to sell, but the restriction does not allow for the former.