chore(script): improve markdown changelog output in sync-react.js (#52052)
This makes it really easy to copy/paste the changelog into the PR description after running `sync-react.js`.
Previously, the output would link to incorrect PRs because it didn't know the PR number was from a different repo.
We know `facebook/react` uses squash so there should always be a PR number at the end of the commit title.
Example of the new format: https://github.com/vercel/next.js/pull/52005