Use consistent method names in admin stats component

We had one method called `vote_count_by_heading` and another one called
`user_count_by_district`, when here districts refer to headings.

app/components/admin/stats/budget_balloting_component.html.erb

@@ -44,7 +44,7 @@
 <table class="investment-projects-summary">
   <th colspan="2"><%= t("admin.stats.budget_balloting.participants_per_district") %></th>
 
-  <% user_count_by_district.each do |heading_name, count| %>
+  <% user_count_by_heading.each do |heading_name, count| %>
     <tr id="user_count_<%= heading_name.parameterize %>">
       <td class="name">
         <%= heading_name %>

app/components/admin/stats/budget_balloting_component.rb

@@ -19,7 +19,7 @@ class Admin::Stats::BudgetBallotingComponent < ApplicationComponent
       budget.lines.group(:heading_id).count.map { |k, v| [Budget::Heading.find(k).name, v] }.sort
     end
 
-    def user_count_by_district
+    def user_count_by_heading
       User.where.not(balloted_heading_id: nil).group(:balloted_heading_id).count.map { |k, v| [Budget::Heading.find(k).name, v] }.sort
     end
 end