In the section Configure connectivity to the core instance, which is about configuring Minion (not core), the instructions specify:

Set configuration settings in the Horizon etc directory. We reference etc relative to the OpenNMS Horizon core home directory. Depending on your operating system, the home directory is /usr/share/opennms for Debian and Ubuntu or /opt/opennms for CentOS and RHEL.

It should read instead:

Set configuration settings in the Minion etc directory. We reference etc relative to the Minion home directory. Depending on your operating system, the home directory is /usr/share/minion for Debian and Ubuntu or /opt/minion for CentOS and RHEL.