media: Documentation: tx-rx: Fix formatting

Fix formatting under "``.enable_streams()`` and ``.disable_streams()``
callbacks" in tx-rx.rst.

Fixes: 30fe661e ("media: Documentation: Deprecate s_stream video op, update docs")
Cc: stable@vger.kernel.org
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>