shill: fix DHCPv6 configuration key typo

DBUS configuration key string for address lease time is incorrect,
so fix it.

BUG=None
TEST=emerge-$BOARD shill

Change-Id: I9ae38c4de49120af83646476093b2b3b88bd75a6
Reviewed-on: https://chromium-review.googlesource.com/281222
Tested-by: Zeping Qiu <zqiu@chromium.org>
Reviewed-by: Paul Stewart <pstew@chromium.org>
Commit-Queue: Zeping Qiu <zqiu@chromium.org>
1 file changed