docs: iio: ad7380: fix supply for ad7380-4

ad7380-4 is the only device from ad738x family that doesn't have an
internal reference. Moreover it's external reference is called REFIN in
the datasheet while all other use REFIO as an optional external
reference. Update documentation to highlight this.

Fixes: 3e82dfc8 ("docs: iio: new docs for ad7380 driver")
Reviewed-by: David Lechner <dlechner@baylibre.com>
Signed-off-by: Julien Stephan <jstephan@baylibre.com>
Link: https://patch.msgid.link/20241022-ad7380-fix-supplies-v3-5-f0cefe1b7fa6@baylibre.com


Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>