media: v4l2-event.rst: adjust table to fit on PDF output

The tables there don't quite fit on PDF output.

Adjust it by adding a tabularcolumns macro.
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

Documentation/media/kapi/v4l2-event.rst

@@ -67,6 +67,8 @@ type).
 
 The ops argument allows the driver to specify a number of callbacks:
 
+.. tabularcolumns:: |p{1.5cm}|p{16.0cm}|
+
 ======== ==============================================================
 Callback Description
 ======== ==============================================================