/usr/share/doc/percona-xtradb-cluster-5.7-dbg/changelog.Debian.gz is in percona-xtradb-cluster-5.7-dbg 5.7.20-29.24-0ubuntu2.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/doc/percona-xtradb-cluster-5.7-dbg/../percona-xtradb-cluster-server-5.7/changelog.Debian.gz