Update source of `secret_key_base` in cookie rotator example
Update source of `secret_key_base` in cookie rotator example

`secrets.secret_key_base` can be one of the many different places where `secret_key_base` is set and therefore it might not work with your application.
Updating the docs to use `Rails.application.secret_key_base` fixes the issue.

Rails `secret_key_base` - [docs](https://api.rubyonrails.org/classes/Rails/Application.html#method-i-secret_key_base)
rails/rails,yahonda/rails,kmcphillips/rails,MSP-Greg/rails,yawboakye/rails,yahonda/rails,georgeclaghorn/rails,Erol/rails,Edouard-chin/rails,Erol/rails,BlakeWilliams/rails,rails/rails,yawboakye/rails,esparta/rails,yahonda/rails,Edouard-chin/rails,Erol/rails,yawboakye/rails,Edouard-chin/rails,kmcphillips/rails,yawboakye/rails,georgeclaghorn/rails,MSP-Greg/rails,georgeclaghorn/rails,rails/rails,BlakeWilliams/rails,esparta/rails,esparta/rails,BlakeWilliams/rails,georgeclaghorn/rails,esparta/rails,BlakeWilliams/rails,MSP-Greg/rails,Edouard-chin/rails,kmcphillips/rails,kmcphillips/rails,rails/rails,yahonda/rails,MSP-Greg/rails,Erol/rails