feat(functions): add missing region tag for documentation

[iennae]

Description

replaces #13557

docs is including requirements in documentation. new sample has been created, but the requirements region tag hasn't moved so it's pulling in the wrong information. This creates new region tag that @adammil2000 can use in documentation.

[snippet-bot]

Here is the summary of changes.

You are about to add 1 region tag.

• functions/billing_stop_on_notification/requirements.txt:1, tag functions_billing_stop_requirements

---

This comment is generated by snippet-bot.
If you find problems with this result, please file an issue at:
https://github.com/googleapis/repo-automation-bots/issues.
To update this comment, add snippet-bot:force-run label or use the checkbox below:

• Refresh this comment