ABI: testing: fix admv8818 attr description

Fix description of the filter_mode_available attribute by pointing to
the correct name of the attribute that can be written with valid values.

Fixes: bf92d87d ("iio:filter:admv8818: Add sysfs ABI documentation")
Signed-off-by: Antoniu Miclaus <antoniu.miclaus@analog.com>
Link: https://patch.msgid.link/20240702081851.4663-1-antoniu.miclaus@analog.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>