Update date for pdf RN 18 (#897)

This PR updates the date to correct release date.

Author: Andriciuc

docs/templates/pdf_cover_page.tpl

@@ -7,6 +7,6 @@
 {% if config.site_description %}
 <h1>{{ config.site_description }}</h1>
 {% endif %} 
-<h2>18.1.1 (November 15, 2025)</h2>
+<h2>18.1.1 (November 28, 2025)</h2>
 <!--<h3>Percona Technical Documentation Team</h3>-->