docs: document ISO 8601 refreshesAt timestamp

[devin-ai-integration]

Summary

Update the GET /v1/account/allowances/daily docs to reflect that refreshesAt is now returned as an ISO 8601 timestamp string.

Link to Devin session: https://app.devin.ai/sessions/170a69643cb8475fa04131218b857171
Requested by: @aakarim

[devin-ai-integration]

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