dt-bindings: iio: adc: stm32-adc: fix id relative path

Fix id relative path that shouldn't contain 'bindings', as pointed out
when submitting st,stm32-dac bindings conversion to json-schema [1].
[1] https://patchwork.ozlabs.org/patch/1257568/



Fixes: a8cf1723 ("dt-bindings: iio: adc: stm32-adc: convert bindings to json-schema")

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