Fix page counting in Slack CI report script.
# What does this PR do?

We started having an issue getting slack report for our CI. The error shown at the end indicates there is an issue to get all the job links for a workflow run. I take a look and find a change is necessary.

I am not sure why it was that way, especially for `i+2` part. But I remembered it has to be that to avoid duplicated pages. Maybe GitHub Actions change their API now and causes the issue.


```bash
    "url": f"{github_actions_job_links['Extract warnings in CI artifacts']}",
KeyError: 'Extract warnings in CI artifacts'
```
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_21795). All of your documentation changes will be reflected on that endpoint.<|||||>My self doubt sprit and intuition led me to look the issue again, and it turned out the issue was coming from the GitHub API call rate limit was reached as we didn't use a token when making these calls. PR #21823 was opened and merged.

The current way of page counting was good - the first page could be `0`, `1` or without page number. The next one would be `2`. My math capability was reduced a lot, especially when I tried to quickly fix things on Friday.