Update README with new prelude

[proppy]

Instead of

Replace SSID and PASSWORD with WiFi credentials
Replace example.firebaseio.com with the Firebase Hostname from step 2.
Replace secret_or_token with Firebase Secret from step 5.
Click Sketch > Upload

It should uses:

#define FIREBASE_HOST "example.firebaseio.com"
#define FIREBASE_AUTH "token_or_secret"
#define WIFI_SSID "SSID"
#define WIFI_PASSWORD "PASSWORD"