media: format.rst: use the right markup for important notes

There's an important note there, but it is not using the
ReST markup. So, it doesn't get any visual highlight on
the output.
Signed-off-by: NMauro Carvalho Chehab <mchehab@s-opensource.com>

Documentation/media/uapi/v4l/format.rst

@@ -78,7 +78,7 @@ output devices is available. [#f1]_
 The :ref:`VIDIOC_ENUM_FMT` ioctl must be supported
 by all drivers exchanging image data with applications.
 
-    **Important**
+.. important::
 
     Drivers are not supposed to convert image formats in kernel space.
     They must enumerate only formats directly supported by the hardware.