Admin section: make the PDF export section pass webstandard tests

[ERROR] org.xwiki.test.webstandards.framework.DefaultValidationTest.null
[ERROR]   Run 1567: DefaultValidationTest.testDocumentValidity:223 Validation errors in /xwiki/bin/admin/XWiki/XWikiPreferences?editor=globaladmin&section=export.pdf
ERROR: No space between attributes. at line [730] column [20]

Extracts of the invalid content:

  728	<select id="pdfGenerator">
  729	<option value="userBrowser"
  730	selected="selected"title="The PDF is generated client-side using the print function provided by the user&#39;s own web browser.">
  731	User Browser
  732	</option>