docs: update security_groups.md tabulation

[mtulio]

Issue

Tabulation is not rendered correctly in the SG page: https://kubernetes-sigs.github.io/aws-load-balancer-controller/latest/deploy/security_groups/

Description

Updating the documentation of security group with required tabulation to make mkdocs render correct the sub items.

Before (current docs):

This PR:

Checklist

• Added tests that cover your change (if possible)
• Added/modified documentation as required (such as the README.md, or the docs directory)
• Manually tested
• Made sure the title of the PR is a good description that can go into the release notes

BONUS POINTS checklist: complete for good vibes and maybe prizes?! 🤯

• Backfilled missing tests for code in same general area 🎉
• Refactored something and made the world a better place 🌟

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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