make_dev_ssd.sh: Correct root partition regular expression

Discovered by CL:2353632, the regular expression for extracting rootfs
partition should include non-digit character first otherwise we won't
get correct number when the partition number is longer than one digit
(e.g., >=10).

BUG=None
TEST=./make_dev_ssd.sh
BRANCH=none

Change-Id: I155e04beec47c55df4d09cb78168ab0a7407c697
Signed-off-by: Hung-Te Lin <hungte@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/2353776
Reviewed-by: Kuang-che Wu <kcwu@chromium.org>