You need to add CDATA tags for the script like this:
<script type="text/javascript">
//<![CDATA[
window.jQuery || document.write("<script src='assets/js/jquery-2.0.3.min.js'>"+"<"+"/script>");
//]]>
</script>
The tags are telling thymeleaf's xml parser that the code between should not be interpreted as XML markup.
This is no longer necessary since Thymeleaf 3.0