GEMS17 — Update documentation around utime to refer to ISO 8601

The time format in the specification is a variant of ISO 8601 (not RFC 3339). Recommend showing time format string as a reference to ISO 8601 and using "D" and fractional in alignment with ISO 8601 rather than alternate definition within the specification.
See https://en.wikipedia.org/wiki/ISO_8601#:~:text=It%20is%20represented%20as%20%22YYYY,as%20%221981%2D095%22.

Update utime documentation

Update the description of the "utime" encoding for both XML and ASCII PSMs to clarify that it maps to the ISO 8601 ordinal date format.
Please refer to the attached Word document that includes these updates with Track Changes enabled.