Added constant instancitation in command line inside MNIST example documentation

doc/mnist.rst

@@ -165,6 +165,16 @@ for which nnenum is sure that the model ``MNIST_256_2.onnx`` is not robust with
 respect to :math:`1 \%` perturbation. At the moment, CAISAR is not able to tell
 which are the images in the dataset that cause such result.
 
+For more flexibility, it is possible to define the concrete values from the
+command line. See the ``--define`` option. For instane, to verify the aforementioned `mnist.why
+<https://git.frama-c.com/pub/caisar/-/blob/master/examples/mnist/mnist.why>`_
+file, the command line is the following:
+
+.. code-block:: console
+
+   $ caisar verify -p nnenum --define model_filename:nets/MNIST_256_2.onnx --define dataset_filename:csv/mnist_test.csv examples/mnist/mnist.why
+   [caisar] Goal robustness: Invalid
+
 
 .. [LiDeng2012] Li Deng, *The MNIST Database of Handwritten Digit Images for
                Machine Learning Research*, IEEE Signal Process. Mag., 2012, pp.