Appease rubocop

Rubocop wants double quoted strings, not single. I missed this when I
merged rails#37601

Author: eileencodes

railties/test/application/rake/multi_dbs_test.rb

@@ -238,7 +238,7 @@ def generate_models_for_animals
               Book.first
             end
           RUBY
-          app_file('Rakefile', dummy_task, 'a+')
+          app_file("Rakefile", dummy_task, "a+")
 
           generate_models_for_animals