Update drop-down wording for event creation pop-up window

Feedback from Fei: 
In the “Create new event” window, a drop-down menu for status shows 4 options. Could you please change the last one from “Completed Missions” to “Completed”?

<img width="451" alt="create new event" src="https://user-images.githubusercontent.com/25951555/47301606-29e0f280-d617-11e8-933c-97bb9ba3f9b8.png">
