tests/acceptance: fix doc reference to avocado_qemu directory

tests/acceptance: fix doc reference to avocado_qemu directory

The "this directory" reference is misleading and confusing, it's a
leftover from when this text was proposed in a README file inside
the "tests/acceptance/avocado_qemu" directory.

When that text was moved to the top level docs directory, the
reference was not updated.

Signed-off-by: Cleber Rosa <crosa@redhat.com>
Reviewed-by: Caio Carrara <ccarrara@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Message-Id: <20190312171824.5134-4-crosa@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>