Small change in the Daisy3 OPF to be compliant with the specifications.

Quote from the specs:
"For upwards compatibility, the element dc-metadata in an OEB package is required to have an attribute of
xmlns:dc="http://purl.org/dc/elements/1.1/"
and
xmlns:oebpackage="http://openebook.org/namespaces/oeb-package/1.0/".