Use rfc6761 example.com domain instead of domain.com in examples

htdocs/core/class/CMailFile.class.php

htdocs/core/class/conf.class.php

htdocs/core/lib/admin.lib.php

htdocs/core/lib/functions.lib.php

htdocs/core/lib/functions2.lib.php

htdocs/core/modules/mailings/example.modules.php

htdocs/core/modules/modMailmanSpip.class.php