[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