dt-bindings: iio: sigma-delta-modulator: fix unit-address in example

Device tree compiler gives a warning if a device node has "@"
but no reg property.
Fix the example in iio: adc: sigma-delta-modulator.

Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>