Improve channel docs and example code (#5898)

* fix Step 2, template filename

* fix comment/example heex template filename to hold the script tag

guides/real_time/channels.md

@@ -410,7 +410,7 @@ Now our `conn.assigns` contains the `current_user` and `user_token`.
 
 ### Step 2 - Pass the Token to the JavaScript
 
-Next, we need to pass this token to JavaScript. We can do so inside a script tag in `lib/hello_web/components/layouts/app.html.heex` right above the app.js script, as follows:
+Next, we need to pass this token to JavaScript. We can do so inside a script tag in `lib/hello_web/components/layouts/root.html.heex` right above the app.js script, as follows:
 
 ```heex
 <script>window.userToken = "<%= assigns[:user_token] %>";</script>

priv/templates/phx.gen.socket/socket.js

@@ -32,7 +32,7 @@ let socket = new Socket("/socket", {params: {token: window.userToken}})
 //     end
 //
 // Now you need to pass this token to JavaScript. You can do so
-// inside a script tag in "<%= web_prefix %>/templates/layout/app.html.heex":
+// inside a script tag in "<%= web_prefix %>/templates/layout/root.html.heex":
 //
 //     <script>window.userToken = "<%%= assigns[:user_token] %>";</script>
 //