media: vidioc-g-dv-timings.rst: fix wrong porch

It is the vfrontporch of field 1 that is one half-line longer,
not the backporch.

The order of the vertical signals in an interlaced system is:

Field 1:

vsync
vbackporch
active video of field 1
vfrontporch + 0.5

Field 2:

il_vsync
il_vbackporch - 0.5
active video of field 2
il_vfrontporch

Interlaced systems that use HALF_LINE set the il_ fields as follows:

il_vfrontporch = vfrontporch
il_vsync = vsync
il_vbackporch = vbackporch + 1

So the total vertical blanking for field 1 is:

	vsync + vbackporch + vfrontporch + 0.5

and for field 2:

	vsync + vbackporch + 1 + vfrontporch - 0.5 ==
	vsync + vbackporch + vfrontporch + 0.5

So each field has the same number of half-lines.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>