Documentation/driver-api: iio/buffers: drop doubled word

Drop the doubled word "struct".

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: linux-doc@vger.kernel.org
Cc: William Breathitt Gray <vilhelm.gray@gmail.com>
Cc: linux-iio@vger.kernel.org
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>