Fix to have LastMod time show up (Category DateLastComment --> DateUpdated)

jwolter

I had to change

          'LastMod' => $Category['DateLastComment'], // line 107 in class.sitemaps.plugin.php

to

            'LastMod' => $Category['DateUpdated'],  // updated this to correct bug with above column incorrect name.

Since the category table did not have the 1st column name.