Fix update PR body script (#3181)

<!--
Open the PR up as a draft until you feel it is ready for a proper
review.

Do not make PR:s from your own `main` branch, as that makes it difficult
for reviewers to add their own fixes.

Add any improvements to the branch as new commits to make it easier for
reviewers to follow the progress. All commits will be squashed to a
single commit once the PR is merged into `main`.

Make sure you mention any issues that this PR closes in the description,
as well as any other related issues.

To get an auto-generated PR description you can put "copilot:summary" or
"copilot:walkthrough" anywhere.
-->

### What

The script works, but it accumulates a bunch of comments:


![image](https://github.com/rerun-io/rerun/assets/1665677/61ad1311-6158-4fa3-81f9-fa2f27f823f2)

### Checklist
* [x] I have read and agree to [Contributor
Guide](https://github.com/rerun-io/rerun/blob/main/CONTRIBUTING.md) and
the [Code of
Conduct](https://github.com/rerun-io/rerun/blob/main/CODE_OF_CONDUCT.md)
* [x] I've included a screenshot or gif (if applicable)
* [x] I have tested [demo.rerun.io](https://demo.rerun.io/pr/3181) (if
applicable)

- [PR Build Summary](https://build.rerun.io/pr/3181)
- [Docs
preview](https://rerun.io/preview/eaaa3695c04a8a81a169a64e29fd6a6fde481e19/docs)
<!--DOCS-PREVIEW-->
- [Examples
preview](https://rerun.io/preview/eaaa3695c04a8a81a169a64e29fd6a6fde481e19/examples)
<!--EXAMPLES-PREVIEW-->
- [Recent benchmark results](https://ref.rerun.io/dev/bench/)
- [Wasm size tracking](https://ref.rerun.io/dev/sizes/)

scripts/ci/update_pr_body.py

@@ -62,7 +62,7 @@ def main() -> None:
 
     examples_preview_link_end = new_body.find(EXAMPLES_PREVIEW_MARKER)
     if examples_preview_link_end != -1:
-        len(EXAMPLES_PREVIEW_MARKER)
+        examples_preview_link_end += len(EXAMPLES_PREVIEW_MARKER)
         examples_preview_link_start = new_body.rfind("\n", 0, examples_preview_link_end) + 1
         new_body = (
             new_body[:examples_preview_link_start] + EXAMPLES_PREVIEW_BARE_LINK + new_body[examples_preview_link_end:]