doc-rst: media-ioc-enum-entities: better format the table

Add a :widths: at the flat-table, to improve the visual.
Signed-off-by: NMauro Carvalho Chehab <mchehab@s-opensource.com>

Documentation/linux_tv/media/mediactl/media-ioc-enum-entities.rst

@@ -54,6 +54,7 @@ id's until they get an error.
 .. flat-table:: struct media_entity_desc
     :header-rows:  0
     :stub-columns: 0
+    :widths: 1 1 1 1 8
 
 
     -  .. row 1