iio: ABI: Fix name of timestamp sysfs file

The name of the file is "current_timetamp_clock" not
"timestamp_clock".

Fixes: bc2b7dab62 ("iio:core: timestamping clock selection support")
Cc: Gregor Boirie <gregor.boirie@parrot.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/ABI/testing/sysfs-bus-iio

@@ -32,7 +32,7 @@ Description:
 		Description of the physical chip / device for device X.
 		Typically a part number.
 
-What:		/sys/bus/iio/devices/iio:deviceX/timestamp_clock
+What:		/sys/bus/iio/devices/iio:deviceX/current_timestamp_clock
 KernelVersion:	4.5
 Contact:	linux-iio@vger.kernel.org
 Description: