Modern Front-End Development for Rails: Typo/Correction? import * as Channels for application.ts

[msducheminjr]

This works without the changes on 6.1.1 and Ruby 3.0.0 (and probably 2.7.x) as-of January 20, 2021. The intro (beta 6 as-of August 2020) said to use edge Rails (6.1 prerelease on master), and it might have worked on master before 6.1.x officially released.