[firebase_crashlytics] Update README code example

[ninnepinne]

Seems like the API has changed but not the code example.
reportInDevMode -> enableInDevMode

Description

README change. Code example

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[ninnepinne]

I signed it!

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CLAs look good, thanks!

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[bparrishMines]

LGTM

Thanks for the contribution! I'll merge once tests pass again.