Update "Eagerly Requiring Helpers" section in testing docs [ci skip]

This will change the path for requiring helper files to be consistent
with previous examples in "Using Separate Files" section

guides/source/testing.md

@@ -1443,7 +1443,7 @@ You may find it convenient to eagerly require helpers in `test_helper.rb` so you
 
 ```ruby
 # test/test_helper.rb
-Dir[Rails.root.join('lib', 'test', '**', '*.rb')].each { |file| require file }
+Dir[Rails.root.join('test', 'test_helpers', '**', '*.rb')].each { |file| require file }
 ```
 
 This has the downside of increasing the boot-up time, as opposed to manually requiring only the necessary files in your individual tests.