Fixing YML in review app docs and screenshot

What does this MR do?

- There's a screenshot
https://docs.gitlab.com/ee/ci/review_apps/#enable-review-apps-button
with incorrect YML. The incorrect YML was also inside the application.
This commit fixes the YML and the screenshot so that it will pass the
CI linting.

This is a docs only change. However one of the docs that I am updating is in the application proper.

Screenshots

The "enable review" button is the button that I am changing the modal for.