ERD2WEditToOneRelationship advertises isMandatory key

This component hasn't honoured the `isMandatory` key since 2006, as discussed on the mailing list in [2010](http://permalink.gmane.org/gmane.comp.web.webobjects.wonder-disc/15336), 2013. This one liner pulls the corresponding `d2wKey` from the Javadocs.