EM-645 Made update to datetime description in specification

[dlzhry2nhs]

Summary

• Routine Change
  See https://github.com/NHSDigital/multicast-notification-service-infrastructure/pull/572 this brings the description into line with the latest changes,

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Reviews Required

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Review Checklist

ℹ️ This section is to be filled in by the reviewer.

• I have reviewed the changes in this PR and they fill all or part of the acceptance criteria of the ticket, and the code is in a mergeable state.
• If there were infrastructure, operational, or build changes, I have made sure there is sufficient evidence that the changes will work.
• I have ensured the changelog has been updated by the submitter, if necessary.