Update outdated README.md factory example

[RyanofWoods]

Summary

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

• https://github.com/solidusio/solidus_paypal_braintree/pull/289/commits/f9dc06d8a97fb23ad61959142c5d49d6a6960a89

Checklist

Check out our PR guidelines for more details.

The following are mandatory for all PRs:

• [x] I have written a thorough PR description.
• [x] I have kept my commits small and atomic.
• [x] I have used clear, explanatory commit messages.

The following are not always needed (~cross them out~ if they are not):

- [ ] I have added automated tests to cover my changes. - [ ] I have attached screenshots to demo visual changes. - [ ] I have opened a PR to update the guides. - [ ] I have updated the README to account for my changes.

[RyanofWoods]

I think it would be good to say that this file moved in the changelog @kennyadsl as it will cause tests to fail for developers.

[RyanofWoods]

Pending - I actually don't think this the way we want users to include extension's factories anymore. Also, this is outdated: https://github.com/solidusio/solidus_paypal_braintree/blame/29136274ecdbe2c554ef26c80074a92567cd7da1/lib/solidus_paypal_braintree/factories.rb

[kennyadsl]

@RyanofWoods you are right, what about fixing it in this same PR?

[RyanofWoods]

Can do @kennyadsl. I will investigate this more this week 🙂