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) Nginx, Inc.
  -->

X:stylesheet {

X:output method="text";

X:template = "link" {
    !{@id}
    X:text{ }
    !{substring-before(@doc, '.xml')}
    X:text{.html#}
    !{@id}
    X:text{;&#10;}
}

}