docs: iio: fix example formatting

This line is part of the code snippet, so it has to be nested in order
to be rendered correctly.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Link: https://lore.kernel.org/r/20210215143511.25471-2-luca@lucaceresoli.net


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