Fix property name of return to

Author: hansott

articles/quickstart/spa/react/_includes/_centralized_login.md

@@ -138,7 +138,7 @@ export default class Auth {
     localStorage.removeItem('isLoggedIn');
 
     this.auth0.logout({
-      return_to: window.location.origin
+      returnTo: window.location.origin
     });
 
     // navigate to the home route