[media] docs-rst: media: Explicitly refer to sub-sampling in scaling documentation

Skipping, which is sometimes used in terminology related to sensors when
referring to sub-sampling is replaced by more suitable sub-sampling
instead. Skipping is retained in a note in parentheses.

Suggested-by: Russell King <rmk+kernel@armlinux.org.uk>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>