README: correct path to Koji Hub configuration file
README: correct path to Koji Hub configuration file

The Koji Hub configuration file is in a sub-directory, not in the
top-level /etc directory.

Signed-off-by: Ken Dreyer <940dddfbf8b0685439b7ecf27daa0ba58b359a2c@redhat.com>

release-engineering/koji-containerbuild,release-engineering/koji-containerbuild