factory_install: Fix typo in device file name (/dev/null).

There was a typo in NULL device (/dev/null) name.

BUG=none
TEST=none

Change-Id: Icf880bcd15dccf12015f0aa31e89495e6b85766a
Reviewed-on: https://chromium-review.googlesource.com/254966
Tested-by: Hung-Te Lin <hungte@chromium.org>
Reviewed-by: Mao Huang <littlecvr@chromium.org>
Commit-Queue: Hung-Te Lin <hungte@chromium.org>
1 file changed