xkb: Fix parsing of XkbSA_DeviceValuator action type

The xkb spec defines that the 7th element of the DeviceValuator key
action description is "valuator 2 value".

This error most likely was accidentally introduced as a copy-paste error
in edeb033f.

Signed-off-by: Povilas Kanapickas <povilas@radix.lt>
4 jobs for fix-xkb-device-valuator-read in 5 minutes and 24 seconds
Status Job ID Name Coverage
  Docker Image
passed #8434574
debian-buster

00:00:17

 
  Build And Test
passed #8434575
autotools

00:05:06

passed #8434576
meson

00:02:33

passed #8434577
meson-noglamor

00:02:21