clarify datetime format is ISO 8601

[jonathanstanley]

Small tweak to clarify docs for string.datetime(). current wording is

"defaults to UTC validation"

however, zod actually isn't validating javascript UTC string format (the following will not succeed with z.string().datetime()) ❌:

const utcDateTime = new Date().toUTCString(); // Mon, 21 Aug 2023 20:42:52 GMT

clarification to show that it is expecting ISO 8601 with no UTC offset. ex ✅:

const isoDateTime = new Date().toISOString(); // 2023-08-21T20:47:22.189Z

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[colinhacks]

Thanks!