Polish Books is not a third party plugin, it's factory fitted, so I've moved this to Library Management.
You have to change the code in the <calibre config>/resources/jacket/template.xhtml:
Code:
<tr>
<!-- If you do not want the series number to be formatted using roman numerals change {series.roman} to {series} -->
<td class="cbj_series" colspan="2">{series}</td>
</tr>
BR