add security group references

[rushmash91]

closes aws-controllers-k8s/community#2456

Description of changes:

• add ref for securitygroups for cachecluster

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[michaelhtm]

Nice 👍
/lgtm

[ack-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: michaelhtm, rushmash91

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[rushmash91]

/retest

[ack-prow]

@rushmash91: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
elasticache-verify-attribution	413b42a	link	false	/test elasticache-verify-attribution

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