iio: Update unit of the voltage scale in the documentation

The documentation says that the result of raw * scale should be in microvolts,
but in reallity all drivers actually report the scale so that the result is in
millivolts. So update the documentation to match reallity.
Signed-off-by: NLars-Peter Clausen <lars@metafoo.de>
Signed-off-by: NJonathan Cameron <jic23@kernel.org>

Documentation/ABI/testing/sysfs-bus-iio

@@ -79,7 +79,7 @@ Description:
 		correspond to externally available input one of the named
 		versions may be used. The number must always be specified and
 		unique to allow association with event codes. Units after
-		application of scale and offset are microvolts.
+		application of scale and offset are millivolts.
 
 What:		/sys/bus/iio/devices/iio:deviceX/in_voltageY-voltageZ_raw
 KernelVersion:	2.6.35
@@ -90,7 +90,7 @@ Description:
 		physically equivalent inputs when non differential readings are
 		separately available. In differential only parts, then all that
 		is required is a consistent labeling.  Units after application
-		of scale and offset are microvolts.
+		of scale and offset are millivolts.
 
 What:		/sys/bus/iio/devices/iio:deviceX/in_capacitanceY_raw
 KernelVersion:	3.2