Update IRC network in the documentation

KDE has moved from freenode to Libera Chat, update the okular manpage to
mention the new server.

doc/man-okular.1.docbook

@@ -106,7 +106,7 @@ is aimed to show documents in different formats.</para>
 </refsect1>
 
 <refsect1 id="feedback"><title>Feedback</title>
-<para>On IRC, irc.freenode.net #okular or via email, okular-devel@kde.org or on the web, &lt;<ulink url="https://okular.kde.org/">https://okular.kde.org/</ulink>&gt;.</para>
+<para>On IRC, irc.libera.chat #okular or via email, okular-devel@kde.org or on the web, &lt;<ulink url="https://okular.kde.org/">https://okular.kde.org/</ulink>&gt;.</para>
 </refsect1>
 
 <refsect1>