Update Firebase.cpp

[michaelchien1972]

1. fix for API KEY authentication
2. typo fix

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[michaelchien1972]

I signed it!

[proppy]

@michaelchien1972 I'm curious where you found the ?api_key option documented.

I think we should switch to:
https://firebase.google.com/docs/reference/rest/database/user-auth (assuming we can sanely implement token refresh on the device)

[maherahmed]

@michaelchien1972 Any documentation for the ?api_key ?