Update what-is-security-serverless.mdx to force a build

[scottybollinger]

This is a non-visible change to indicate that the new build pipeline is successful and live on https://www.elastic.co/docs. Safe to merge this once out of draft

[github-actions]

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[mergify]

This pull request does not have a backport label. Could you fix it @scottybollinger? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• v7.x is the label to automatically backport to the 7.x branch.
• v7./d./d is the label to automatically backport to the 7./d branch. /d is the digit

NOTE: backport-skip has been added to this pull request.