Fix memoization in ContributionsCalendar#activity_dates

This doesn't appear to be actually called twice, but having it appear to work
but not would be a problem if it was.

lib/gitlab/contributions_calendar.rb

@@ -28,7 +28,7 @@ module Gitlab
       union = Gitlab::SQL::Union.new([repo_events, issue_events, mr_events, note_events])
       events = Event.find_by_sql(union.to_sql).map(&:attributes)
 
-      @activity_events = events.each_with_object(Hash.new {|h, k| h[k] = 0 }) do |event, activities|
+      @activity_dates = events.each_with_object(Hash.new {|h, k| h[k] = 0 }) do |event, activities|
         activities[event["date"]] += event["total_amount"]
       end
     end