setting "fake" crossmark policy doi

from crossref support staff:

> [...]
> You can't totally omit the <crossmark_policy> element. For historical reasons, it's strictly required by the schema. But, as a workaround, you can just repeat the DOI of the original paper in that field. It's not used for anything, but it has to have a valid DOI in it.

Author: gamboz

src/templates/common/identifiers/crossref_article.xml

@@ -56,7 +56,7 @@
 
             {% if article.erratum_of %}
             <crossmark>
-              <crossmark_policy>{{ crossmark_policy_doi }}</crossmark_policy>
+              <crossmark_policy>{{ article.object.journal|setting:'crossref_prefix' }}/not-used</crossmark_policy>
               <updates>
                 <update type="erratum" date="{{ now|date:"Y-m-d" }}">{{ article.erratum_of.get_doi }}</update>
               </updates>