[media] gspca_pac7302: correct register documentation

R,G,B balance registers are 0x01-0x03 instead of 0x02-0x04,
which lead to the wrong conclusion that values are inverted.
Exposure is controlled via page 3 registers and this is already documented.
Also fix a whitespace issue.

Signed-off-by: Frank Schäfer <fschaefer.oss@googlemail.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>