doc/configuration: UUUDriver: align example with reality

Besides the image name, the UUUDriver supports a script attribute.
This is described in the Arguments: section, but not in the example,
which uses the non-existent cmd attribute instead.

Fix that and while at it, correctly indicate that script is optional.
Omitting it will just remove the -b $script from the uuu invocation.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>

Author: a3f

doc/configuration.rst

@@ -2493,15 +2493,15 @@ Implements:
        drivers:
          UUUDriver:
            image: 'mybootloaderkey'
-           cmd: 'spl'
+           script: 'spl'
 
    images:
      mybootloaderkey: 'path/to/mybootloader.img'
 
 Arguments:
   - image (str): optional, key in :ref:`images <labgrid-device-config-images>` containing the path
     of an image to bootstrap onto the target
-  - script (str): run built-in script with ``uuu -b``, called with image as arg0
+  - script (str): optional, run built-in script with ``uuu -b``, called with image as arg0
 
 USBStorageDriver
 ~~~~~~~~~~~~~~~~