Plugin document report contains invalid XML in 'Usage' section.

The 'Usage' section of the plugin documentation report contains invalid XML. Looking at this report on the maven-plugin-plugin itself [1], the "groupId", "artifactId" and "version" closing tags are missing the forward slash. This is a trivial matter, but is frustrating to those that copy/paste this section to add a Maven plug-in to their POM. The attached simple patch corrects the issue.

[1] http://maven.apache.org/plugins/maven-plugin-plugin/plugin-info.html