Doc: Fix code-block syntax bug

Fixes error:
...
doc/installation/installation_linux_steps_optional.rst:252: ERROR:
Error in "code-block" directive:
maximum 1 argument(s) allowed, 5 supplied.

.. code-block:: bash
   $ git clone git://git.qemu-project.org/qemu.git


...

Change-Id: Idcb4f018114298109507814ea3b84eb823e8426e
Signed-off-by: Andrei Emeltchenko <andrei.emeltchenko@intel.com>