media: colorspaces-details.rst: fix V4L2_COLORSPACE_JPEG description

The description was missing V4L2_XFER_FUNC_SRGB in the description
of what V4L2_COLORSPACE_JPEG stands for.

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