Rename subject rdf marker to be more self-explanatory

This renaming makes our morph files more like those of metafacture-core, which uses "~rdf:about" for similar reasons,  see e.g. https://github.com/culturegraph/metafacture-core/blob/42bb838cac10798a5f408eda5a5061b150bdd446/examples/marc21-to-edm/MARC21-EDM.xml