Fix schema mismatch: Use "CalendarItemFormats" (plural) in calendar-items.json

[deralbert]

Description:

This PR fixes a schema inconsistency in web-experiments in calendar-items.json where the code expects CalendarItemFormat (singular), but the schema defines it as CalendarItemFormats (plural).

Issue

• The mismatch causes errors when the code attempts to reference CalendarItemFormat, leading to failures. For example, if one uses the following valid update call:

browser.calendar.items.update(eventItem.calendarId, eventItem.id, {
    format: "jcal",
    item: comp.toJSON()
});

despite all parameters being correct, this results in a DOMException / InvalidStateError.

• The correct name in the schema is CalendarItemFormats (plural), which aligns with the definition of the enum CalendarItemFormats.

Fix

Updated references in calendar-items.json to use CalendarItemFormats consistently.