Reflect spec change in https://github.com/tc39/proposal-intl-relative-time/pull/81

Bug: v8:7992
Cq-Include-Trybots: luci.v8.try:v8_linux_noi18n_rel_ng
Change-Id: Ic2153f8355b8380d2986b34d46eb6bb6771d4b59
Reviewed-on: https://chromium-review.googlesource.com/1154236Reviewed-by: Daniel Ehrenberg <littledan@chromium.org>
Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Commit-Queue: Frank Tang <ftang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54793}