Update category examples to use the _atom_site.structure_id item

[vaitkus]

The current usage example for the ATOM_SITE category was copied verbatim from the CIF_CORE dictionary. This example should be updated to properly reflect the changed usage of the category in the MultiBlock dictionary, e.g. by also including the _atom_site.structure_id data item.

This suggestion was originally raised in a discussion thread of PR #6 (see https://github.com/COMCIFS/MultiBlock_Dictionary/pull/6/files#r1479569970).