Update documentation for ActiveRecord >= 6.1

[lvonk]

In https://www.sequent.io/docs/building-a-web-application.html a code snippet says:

Sequent::ApplicationRecord.configurations[env.to_s] = config.stringify_keys

In ActiveRecord >= 6.1 this should be:

Sequent::ApplicationRecord.configurations = { env.to_s => config.stringify_keys }