iio: drop wrong reference from Kconfig

The documentation lives in drivers/staging/iio/Documentation, but
according to Jonathan it's obsolete and needs fixing.

Signed-off-by: Jiri Kosina <jkosina@suse.cz>
Acked-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>