Update path to correct layout template in 'Channels' guide and socket.js template

[tonydangblog]

The guide for Channels and the socket.js template generated by phx.gen.socket suggest to add the user token in app.html.heex:

// lib/hello_web/components/layouts/app.html.heex

<script>window.userToken = "<%= assigns[:user_token] %>";</script>
<script src={~p"/assets/app.js"}></script>

However, in the latest Phoenix, the app.js script is now located in root.html.heex. This PR updates the guide and template to point to the root.html.heex template.