fix typo in rootfs var name

Copy & paste bites.

BUG=chromium-os:34715
TEST=None

Change-Id: I79c2afac590c2133063bdc394f30edae66abea24
Reviewed-on: https://gerrit.chromium.org/gerrit/33945
Reviewed-by: Liam McLoughlin <[email protected]>
Tested-by: Liam McLoughlin <[email protected]>
Commit-Ready: Mike Frysinger <[email protected]>
Reviewed-on: https://gerrit.chromium.org/gerrit/34027
Reviewed-by: Mike Frysinger <[email protected]>
Tested-by: Chris Masone <[email protected]>
1 file changed