Update :dedent: example

The presence or absence of the ``:dedent: 4`` option in the example

.. code-block:: ruby
   :dedent: 4

       some ruby code

makes no difference for the output due to how ReST handles whitespace.
If it is present, then ReST indent is 3 and the 4 spaces of the code are
stripped by the :dedent: option.  If the dedent option is missing, then
the ReST indent is 7 and all whitespace is stripped again.

Add a dummy option to fixate the ReST indent to 3, so adding and
removing of the option in the example has an effect on the output.

doc/usage/restructuredtext/directives.rst

@@ -598,6 +598,7 @@ __ https://pygments.org/docs/lexers
       are removed via :func:`textwrap.dedent()`.  For example::
 
          .. code-block:: ruby
+            :linenos:
             :dedent: 4
 
                 some ruby code