Configure sitemap format to silence warning

pelicanconf.py

@@ -117,3 +117,5 @@ SECONDARY_LIGHT = "#005EBE"
 SPONSOR_LOGOS = (('/images/sponsors/logo-bareos.png', 'https://www.bareos.com/'),
                  ('/images/sponsors/logo-speedpartner.png', 'https://www.speedpartner.de/'),
                  ('/images/sponsors/logo_informatik-aktuell.png', 'https://www.informatik-aktuell.de/'))
+
+SITEMAP = {'format': 'xml'}