doctrine-mapping.xsd target-entity type

I noticed some problems with
the doctrine-mapping.xsd (which apparently is not used to validate the
mapping, but only provided to support code completion in your xml-
editor)

the target-entity attribute of the different relations is of type
xs:NMTOKEN, while I think this should be STRING: just like the name
attribute of an entity, since the target-entity attribute is the text
that refers to a entity.name and can include backslash for the
namespaces.