staging:iio: remove num_interrupt_lines from documentation

Commit 5aa96188 ("staging:iio: remove broken support for
multiple event interfaces.") removed the num_interrupt_lines field from
struct iio_info but the documentation was never updated.

Signed-off-by: Johan Hovold <jhovold@gmail.com>
Acked-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>