fix: schedule mode color issue of the item.

[charithAmila]

Fixed the issue: #965

Test:
Try to change the background color of the event like below.

<Calendar
  height={SCREEN_HEIGHT}
  events={state.events}
  mode="schedule"
  eventCellStyle={(event: ICalendarEventBase & { color?: string }) => {
    if (event.color) {
      return [{ backgroundColor: event.color }]
    }
    return []
  }}
/>

Result:

[vercel]

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Name	Status	Preview	Comments	Updated (UTC)
react-native-big-calendar	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Nov 8, 2023 5:31am

[acro5piano]

@charithAmila Thanks! It looks fine!