Because its wrong? This page has a simple example http://db.apache.org/jdo/jdoconfig_dtd.html
The documentation of any JDO implementation would explain what that file is for, as would a simple internet search.
A better XML header would be something like
<jdoconfig xmlns="http://xmlns.jcp.org/xml/ns/jdo/jdoconfig"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/jdo/jdoconfig
http://xmlns.jcp.org/xml/ns/jdo/jdoconfig_3_0.xsd" version="3.0">