iio: ABI: Fix typo in in_proximity_raw description

Signed-off-by: NPeter Meerwald <pmeerw@pmeerw.net>
Signed-off-by: NJonathan Cameron <jic23@kernel.org>

Documentation/ABI/testing/sysfs-bus-iio

@@ -1233,7 +1233,7 @@ KernelVersion:	3.4
 Contact:	linux-iio@vger.kernel.org
 Description:
 		Proximity measurement indicating that some
-		object is near the sensor, usually be observing
+		object is near the sensor, usually by observing
 		reflectivity of infrared or ultrasound emitted.
 		Often these sensors are unit less and as such conversion
 		to SI units is not possible. Higher proximity measurements