improve accessibility

app/views/start_form/completed.html.erb

@@ -1,4 +1,4 @@
-<div class="max-w-md mx-auto px-2 mt-12 mb-4">
+<main class="max-w-md mx-auto px-2 mt-12 mb-4">
   <div class="space-y-6 mx-auto">
     <div class="space-y-6">
       <div class="flex items-center justify-center">
@@ -32,5 +32,5 @@
       </toggle-submit>
     <% end %>
   </div>
-</div>
+</main>
 <%= render 'shared/attribution', link_path: '/start', account: @submitter.account %>