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: bf92d87d7c67 ("iio:filter:admv8818: Add sysfs ABI documentation")
Signed-off-by: Antoniu Miclaus <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jonathan Cameron <[e-mail]>

Documentation/ABI/testing/sysfs-bus-iio-filter-admv8818

···
 Contact:	[e-mail]
 Description:
 		Reading this returns the valid values that can be written to the
-		on_altvoltage0_mode attribute:
+		filter_mode attribute:
 
 		- auto -> Adjust bandpass filter to track changes in input clock rate.
 		- manual -> disable/unregister the clock rate notifier / input clock tracking.