Use UTC with the Z timezone indicator in navDates for 230-navdate

[markpbaggett]

Recipe info:

• Name: Navigation by Chronology
• link: https://iiif.io/api/cookbook/recipe/0230-navdate/

Correction request

In the IIIF presentation API, the navDate Section states:

The value must be an XSD dateTime literal. The value must have a timezone, and should be given in UTC with the Z timezone indicator, but may instead be given as an offset of the form +hh:mm.

In this recipe, the value follows the latter, but I think it should be the former (e.g. 1986-01-01T00:00:00+00:00 becomes 1986-01-01T00:00:00Z)

[glenrobson]

Cookbook authors agree this would be a good change