FCM codelab reference obsolete API

[sgserg]

[REQUIRED] Codelab info

https://firebase.google.com/codelabs/firebase-cloud-functions#0

[REQUIRED] Describe the problem

(https://firebase.google.com/codelabs/firebase-cloud-functions#9) references admin.messaging().sendToDevice which seems to be obsolete.

[HAFIDmouhssin]

#5.00
#826