media: docs: Fix mem_offset name in multiplane example

The v4l2_buffer type has an "offset" field, but the corresponding field
in the v4l2_plane type is called "mem_offset". Use the correct name in
this example.

Signed-off-by: Matthijs Kooijman <matthijs@stdin.nl>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>