Documentation: iio: Fix KernelVersion in counter sysfs ABI documentation

The IIO counter driver support did not appear in the 4.9 version of the
Linux kernel. This patch fixes the KernelVersion lines of the relevant
IIO counter sysfs ABI documentation.

Fixes: 1a8f324a ("iio: Implement counter channel type and info constants")
Signed-off-by: William Breathitt Gray <vilhelm.gray@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>