doc: temporary workaround for pytest-pygments lexing error

pytest-pygments doesn't yet recognize the skip reason in summary line
added recently. Workaround it until we get to updating it.

doc/en/fixture.rst

@@ -947,7 +947,7 @@ Example:
 
 Running this test will *skip* the invocation of ``data_set`` with value ``2``:
 
-.. code-block:: pytest
+.. code-block::
 
     $ pytest test_fixture_marks.py -v
     =========================== test session starts ============================