Update testing-webhooks.md

[Deborah-Digges]

Why:

This is a very small issue I noticed while reading the Webhook docs.

What's being changed:

Check off the following:

• All of the tests are passing.
• I have reviewed my changes in staging.
• For content changes, I have reviewed the localization checklist
• For content changes, I have reviewed the Content style guide for GitHub Docs.

[welcome]

Thanks for opening this pull request! A GitHub docs team member should be by to give feedback soon. In the meantime, please check out the contributing guidelines.

[runleonarun]

Thank you @Deborah-Digges! This may be small, but it's great to catch and clarify. We appreciate your help 🥇

[github-actions]

Thanks very much for contributing! Your pull request has been merged 🎉 You should see your changes appear on the site in approximately 24 hours.

If you haven't already, you can add yourself to the list of contributors by creating a new comment in this PR using these instructions. Thanks again! ✨