media: vidioc-g-dv-timings.rst: fix typo (frontporch -> backporch)

The description of V4L2_DV_FL_HALF_LINE mixed up frontporch with backporch.

It's the backporch that has different sizes for interlaced formats, the frontporch
remains constant.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>