iio: hx711: fix spurious unit-address in example

Device tree compiler (dtc) gives a warning if a device node has "@" with a
following number as it's name but no reg property.

Fix the example in the documentation of avia,hx711 to conform to dtc
behavior.

Signed-off-by: Andreas Klinger <ak@it-klinger.de>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>