iio: Documentation: fix kernel version for 4.0 new ABI

Kernel version for new ABI in 4.0 has been documented
as 3.20, since the changes have been merged before the kernel
version number change.

Change kernel version from 3.20 to 4.0.

Signed-off-by: Irina Tirdea <irina.tirdea@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>