iio: adc: vf610: fix conversion mode sysfs node name

The documentation missed the "in_" prefix for this IIO_SHARED_BY_DIR
entry.

Fixes: bf04c1a3

 ("iio: adc: vf610: implement configurable conversion modes")
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Acked-by: Haibo Chen <haibo.chen@nxp.com>
Link: https://lore.kernel.org/r/560dc93fafe5ef7e9a409885fd20b6beac3973d8.1653900626.git.baruch@tkos.co.il
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>