Make generated mojo description valid XHTML

• Patch Submitted:

  Yes

Currently, the PluginXdocGenerator used to create the description of the mojo for inclusion in the site produces something like the following:

<p><b>Mojo Attributes</b>:<ul><li>Requires a Maven 2.0 project to execute.</li></ul></p>

This is no valid XHTML, <p> must not contain <ul>.