Add `WIKI_UPDATED` to `EventType` enum.

[BURG3R5]

In this issue, you'll add a new member to the EventType enum present in the file bot/models/github/event_type.py

The name of the member will be WIKI_UPDATED

The keyword for the member will be "wu"

The "docs" for the member will be "Page(s) in the Wiki were updated"

This issue is a part of a bigger issue (#1). Leave a comment if you have any doubts.