Documentation: kunit: fixed sphinx error in code block

Fix a missing newline in a code block that was causing a warning:

Documentation/dev-tools/kunit/usage.rst:553: WARNING: Error in "code-block" directive:
maximum 1 argument(s) allowed, 3 supplied.

.. code-block:: bash
        modprobe example-test

Signed-off-by: Brendan Higgins <brendanhiggins@google.com>
Reviewed-by: Alan Maguire <alan.maguire@oracle.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>