iio: fix typo, Documentation/ABI/testing/sysfs-bus-iio-frequency-adf4350

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

Documentation/ABI/testing/sysfs-bus-iio-frequency-adf4350

@@ -18,4 +18,4 @@ Description:
 		adjust the reference frequency accordingly.
 		The value written has no effect until out_altvoltageY_frequency
 		is updated. Consider to use out_altvoltageY_powerdown to power
-		down the PLL and it's RFOut buffers during REFin changes.
+		down the PLL and its RFOut buffers during REFin changes.