Update index.md

What does this MR do and why?

Updates the documentation to use Security/Container-Scanning.gitlab-ci.yml instead of Jobs/Container-Scanning.gitlab-ci.yml as it states Jobs/ is empty in 15.7 self-managed

• Related to !103262 (merged)
• Related to #381665 (closed)

Screenshots or screen recordings

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How to set up and validate locally

As requested by user: https://gitlab.zendesk.com/agent/tickets/383946

• Create a blank 15.7 Self-managed project.
• Create a blank .gitlab-ci.yml
• Add:

include:
#     - template: Jobs/Container-Scanning.gitlab-ci.yml
    - template: Security/Container-Scanning.gitlab-ci.yml

• See error for Jobs but not Security

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.