docs: abi: iio: Use What: consistently

Change "[w|W]hat[:| ]" to What: for consistency.

Signed-off-by: Gwendal Grignou <gwendal@chromium.org>
Link: https://lore.kernel.org/r/20200828233156.2264689-2-gwendal@chromium.org
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>