media: doc: pixfmt-yuv: Fix 4:4:4 subsampling info

YUV 4:4:4 is not subsampled, fix this in the docs.

Fixes: da785536

 ("media: doc: pixfmt-yuv: Move all semi-planar YUV formats to common file")
Signed-off-by: Helen Koike <helen.koike@collabora.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>