V4L/DVB: V4L2 Spec: fix V4L2_CTRL_TYPE_CTRL_CLASS documentation

The spec says that the control ID for these types of controls is the same
as that of the control class. But it should read: 'control class + 1'.

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>