media: gspca: fix Kconfig help info

Documentation/video4linux/gspca.txt is missing.
It has moved to Documentation/media/v4l-drivers/gspca-cardlist.rst

Signed-off-by: winton.liu <18502523564@163.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>