doc: include V8 commit URL in V8 backport guide

Use `Commit:` for the V8 commit, and `PR-URL:` for the Node PR URL.

Refs: https://github.com/nodejs/node/pull/16053#issuecomment-334868621
PR-URL: https://github.com/nodejs/node/pull/16054
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/guides/maintaining-V8.md

@@ -196,7 +196,8 @@ Original commit message:
   Review-Url: https://codereview.chromium.org/2159683002
   Cr-Commit-Position: refs/heads/master@{#37833}
 
-PR-URL: <pr link>
+Refs: https://github.com/v8/v8/commit/a51f429772d1e796744244128c9feeab4c26a854
+PR-URL: https://github.com/nodejs/node/pull/7833
 ```
 * Open a PR against the `v6.x-staging` branch in the Node.js repo. Launch the normal and [V8-CI](https://ci.nodejs.org/job/node-test-commit-v8-linux/) using the Node.js CI system. We only needed to backport to `v6.x` as the other LTS branches weren't affected by this bug.