Extract: Fix crrev reference in README.chromium

Bug: 953256
Change-Id: I3f18cdbd6551a3c1e768354c8452cfc6de0d875c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3641229


Commit-Queue: Noel Gordon <noel@chromium.org>
Reviewed-by: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1002527}
NOKEYCHECK=True
GitOrigin-RevId: c09ef1fb4048d7ea2d59543a46b8c73a4ec9d8e6