changed datetime format of published tag

cps/templates/feed.xml

@@ -53,7 +53,7 @@
         <name>{{entry.Books.publishers[0].name}}</name>
       </publisher>
     {% endif %}
-    <published>{{entry.Books.pubdate}}</published>
+    <published>{{entry.Books.pubdate.strftime("%Y-%m-%dT%H:%M:%S+00:00")}}</published>
     {% for lang in entry.Books.languages %}
       <dcterms:language>{{lang.lang_code}}</dcterms:language>
     {% endfor %}