Fix typo in CA-8(2) FR additional guidance

[Rene2mt]

Committer Notes

This PR resolves issue #551 correcting a typo in the "Additional FedRAMP Requirements and Guidance" part of the control.

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• If applicable, have all FedRAMP Documents Related to OSCAL Adoption affected by the changes in this issue have been updated.?
• If applicable, does this PR reference the issue it addresses and explain how it addresses the issue?

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[david-waltermire]

This change looks good. Since this represents a very small non-controversial, editorial change I am in favor of merging this ASAP.