verify-sof-firmware-load: fix error message according to new test

Fixes commit cd2d707 that now looks for the "firmware boot
complete" message instead of the firmware version(s) but forgot to
update the error message.

Signed-off-by: Marc Herbert <marc.herbert@intel.com>

Author: marc-hb

test-case/verify-sof-firmware-load.sh

@@ -35,5 +35,5 @@ else # failed, show some logs
     journalctl_cmd --lines 50 || true
     printf ' ------\n  Check journalctl status: \n ---- \n'
     systemctl --no-pager status systemd-journald* || true
-    die "Cannot find the sof audio version"
+    die "Cannot find any 'sof boot complete' message in logs since boot time"
 fi