Added indentation to directive/variables indexes. While these files aren't designed to be read/edited by humans, indentation makes things more convenient. E.g., terminal is no longer flooded by a very long line with full list of directives if dirindex.xml is matched by grep.

<!--
  Copyright (C) Igor Sysoev
  Copyright (C) Nginx, Inc.
  -->

X:stylesheet {

X:param VERSION = "'development'";

X:output method="text";

X:template = "/versions" { !! "download[@tag=$VERSION][1]/item[1]/@ver"; };

}