docs: iio: fix directory naming

This directory is a, well, directory, not a file.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Link: https://lore.kernel.org/r/20210215143511.25471-3-luca@lucaceresoli.net


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>