Update README.md

Since we're comparing two classes here, a case statement is going to call [Module#===](http://www.ruby-doc.org/core-2.0/Module.html#method-i-3D-3D-3D). That will check whether resource_class is an instance of User (which it would never be) when really we just want to see if they are equivalent, which is easily accomplished here by just using an if statement.