Add "administrators" to Vale rule

What does this MR do?

Fix the Admin Vale rule not to flag "administrators" as incorrect.

Related issues

Author's checklist (required)

• Follow the Documentation Guidelines and Style Guide.
• If you have Developer permissions or higher:
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Do not add the feature, frontend, backend, ~"bug", or database labels if you are only updating documentation. These labels will cause the MR to be added to code verification QA issues.

When applicable:

• Update the permissions table.
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• Add GitLab's version history note(s).
• Add/update the feature flag section.

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