Expand the first appearance of the SSOT acronym

Why is this change being made?

Our handbook writing style guidelines (item 36) recommends that acronyms should be avoided, but if we do use one, then the same should be expanded at least once in the document.

This is the first appearance of the SSOT acronym on this page, thus I think it makes sense to expand it.

Author Checklist

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• Added a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
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