Add last_seen_ts to query user example

[turt2live]

This section could probably do with a lot more editorial attention, but for now this is all there is in terms of documentation. The field is already returned by Synapse:

synapse/synapse/handlers/admin.py

Line 150 in 4587dec

user_info_dict["last_seen_ts"] = last_seen_ts

Pull Request Checklist

• Pull request is based on the develop branch
• Pull request includes a changelog file. The entry should:
  • Be a short description of your change which makes sense to users. "Fixed a bug that prevented receiving messages from other servers." instead of "Moved X method from EventStore to EventWorkerStore.".
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• Code style is correct
  (run the linters)