Fix modal text warning in answers documents spec

It looks like we get this warning if we check the dialog message. Using
`accept_confirm` the same way we do in the rest of the application
solves the problem.

spec/features/admin/poll/questions/answers/documents/documents_spec.rb

@@ -34,9 +34,7 @@ describe "Documents" do
     visit admin_answer_documents_path(answer)
     expect(page).to have_content(document.title)
 
-    accept_confirm "Are you sure?" do
-      click_link "Delete"
-    end
+    accept_confirm { click_link "Delete" }
 
     expect(page).not_to have_content(document.title)
   end