iio: ABI: Fix wrong format of differential capacitance channel ABI.

in_ only occurs once in these attributes.

Fixes: 0baf29d6 ("staging:iio:documentation Add abi docs for capacitance adcs.")
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20220626122938.582107-3-jic23@kernel.org