iio: dac: ad5770r: kernel-doc fix case of letter R wrong in structure name

Nothing useful to add beyond this causing a warning with W=1

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Alexandru Ardelean <ardeleanalex@gmail.com>
Link: https://lore.kernel.org/r/20210314164655.408461-4-jic23@kernel.org