media: ipu3.rst: fix a build warning

Sphinx 2.4.4 produces this warning:

	Documentation/admin-guide/media/ipu3.rst:235: WARNING: Title underline too short.

Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>