Update Spree tutorial to account for undefined method assume_from_symbol...

... for Money:Class #4538 error

This saves time for new Spree users that are automatically downloading the Spree gem - version is 2.2.1. When they are not able to load the demo site, they will be able to see a quick solution. 

Ryan addressed the solution in this Github issue: https://github.com/spree/spree/issues/4538.