Documentation: iio: fix a typo

Rename function name to the actual name referenced in
struct iio_sw_trigger_ops.

Signed-off-by: Lars Poeschel <poeschel@lemonage.de>
Link: https://lore.kernel.org/r/20200904091911.269715-1-poeschel@lemonage.de


Signed-off-by: Jonathan Corbet <corbet@lwn.net>