/usr/share/xml/docbook/custom/simple/1.1/sdocbookref-custom.dtd is in docbook-simple 1.1-4.2.
This file is owned by root:root, with mode 0o777.
It is a symlink to /etc/sgml/docbook-simple/1.1/sdocbookref-custom.dtd
/etc/sgml/docbook-simple/1.1/sdocbookref-custom.dtd