Update outdated README.md factory example

These factories were recently moved when making the extension complaint
to solidus_dev_support:
- f9dc06d

README.md

@@ -353,7 +353,7 @@ When testing your applications integration with this extension you may use it's
 Simply add this require statement to your spec_helper:
 
 ```ruby
-require 'solidus_paypal_braintree/factories'
+require 'solidus_paypal_braintree/testing_support/factories.rb'
 ```
 
 Development