[calendar] Fix event move URL

Summary:
According to Google Calendar API (https://developers.google.com/calendar/v3/reference/events/move),
the proper url is https://www.googleapis.com/calendar/v3/calendars/calendarId/events/eventId/move

Test Plan:

1. Move event between calendars in KOrganizer
2. The change gets propagated to Google Calendar Web UI

Reviewers: dvratil

Reviewed By: dvratil

Subscribers: kde-pim

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D28160