Rename event_types variable to event_names

This is consistent with the `name` column in `ahoy_events`.

app/controllers/admin/stats_controller.rb

@@ -1,6 +1,6 @@
 class Admin::StatsController < Admin::BaseController
   def show
-    @event_types = Ahoy::Event.distinct.order(:name).pluck(:name)
+    @event_names = Ahoy::Event.distinct.order(:name).pluck(:name)
 
     @visits    = Visit.count
     @debates   = Debate.with_hidden.count

app/views/admin/stats/show.html.erb

@@ -114,7 +114,7 @@
 
       </div>
 
-      <% @event_types.each do |event| %>
+      <% @event_names.each do |event| %>
         <h3>
           <%= link_to graph_link_text(event),
                       graph_admin_stats_path(event: event) %>