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: NCleber Rosa <crosa@redhat.com>
Reviewed-by: NCaio Carrara <ccarrara@redhat.com>
Reviewed-by: NPhilippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: NCornelia Huck <cohuck@redhat.com>
Message-Id: <20190312171824.5134-4-crosa@redhat.com>
Signed-off-by: NEduardo Habkost <ehabkost@redhat.com>

docs/devel/testing.rst

@@ -590,8 +590,9 @@ Alternatively, follow the instructions on this link:
 Overview
 --------
 
-This directory provides the ``avocado_qemu`` Python module, containing
-the ``avocado_qemu.Test`` class.  Here's a simple usage example:
+The ``tests/acceptance/avocado_qemu`` directory provides the
+``avocado_qemu`` Python module, containing the ``avocado_qemu.Test``
+class.  Here's a simple usage example:
 
 .. code::