Step 10 cannot send notification request: The request was missing an Authentication Key

### [REQUIRED] Codelab info

https://codelabs.developers.google.com/codelabs/firebase-web/#10

### [REQUIRED] Project setup

As described in the codelab.

### [REQUIRED] Describe the problem

Broken instructions.
When attempting to send the notification HTTP request as described, the following response arrives:
`HTTP/1.1 401 The request was missing an Authentication Key (FCM Token). Please, refer to section "Authentication" of the FCM documentation, at https://firebase.google.com/docs/cloud-messaging/server.`

I send both the Authorization header, and the Device token.
Also, there is no "Authentication" section in the linked documentation.