Update readme.md under messaging quickstart with new FCM REST API

[leandersonralves]

Please, update link to send a push notification from command line curl.

not working: https://android.googleapis.com/gcm/send

working: https://fcm.googleapis.com/fcm/send

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[patm1987]

This bug report is for the file messaging/testapp/readme.md. Two samples there are:

curl --header "Authorization: key=<Server Key>" --header "Content-Type: application/json" https://android.googleapis.com/gcm/send -d '{"notification":{"title":"Hi","body":"Hello from the Cloud"},"data":{"score":"lots"},"to":"<Registration Token>"}'

and

curl --header "Authorization: key=<Server Key>" --header "Content-Type: application/json" https://android.googleapis.com/gcm/send -d '{"notification":{"title":"Hi","body":"Hello from the Cloud"},"data":{"score":"lots"},"to":"/topics/<Topic>"}'

It appears that they should be:

curl --header "Authorization: key=<Server Key>" --header "Content-Type: application/json" https://fcm.googleapis.com/fcm/send -d '{"notification":{"title":"Hi","body":"Hello from the Cloud"},"data":{"score":"lots"},"to":"<Registration Token>"}'

and

curl --header "Authorization: key=<Server Key>" --header "Content-Type: application/json" https://fcm.googleapis.com/fcm/send -d '{"notification":{"title":"Hi","body":"Hello from the Cloud"},"data":{"score":"lots"},"to":"/topics/<Topic>"}'

Which would match the reference docs for the Legacy HTTP Server Protocol.

We may want to take this a step forward and see if we can use the test interface here.