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>