media: Documentation: staging/ipu3-imgu: Fix reference file name

The intel-ipu3.h intended-to-be-uAPI header is currently under
drivers/staging/media/ipu3/include/, not include/uapi/linux.

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