media: Documentation: media: Fix recently introduced build warning in subdev docs

A reference to the sub-device pad ops was not follwed by a whitespace,
resulting in a warning during documentation build. Fix it.
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Fixes: 25c8d9a7 ("media: Documentation: v4l: Document that link_validate op is valid for sink only")
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

Documentation/driver-api/media/v4l2-subdev.rst

@@ -123,7 +123,7 @@ Don't forget to cleanup the media entity before the sub-device is destroyed:
 	media_entity_cleanup(&sd->entity);
 
 If a sub-device driver implements sink pads, the subdev driver may set the
-link_validate field in :c:type:`v4l2_subdev_pad_ops`to provide its own link
+link_validate field in :c:type:`v4l2_subdev_pad_ops` to provide its own link
 validation function. For every link in the pipeline, the link_validate pad
 operation of the sink end of the link is called. In both cases the driver is
 still responsible for validating the correctness of the format configuration