edit Readme.md (#762)
edit Readme.md (#762)

typo : 
PROVDER -> PROVIDER
samtstern/FirebaseUI-Android,firebase/FirebaseUI-Android,JosefHruska/FirebaseUI-Android,SUPERCILEX/FirebaseUI-Android,firebase/FirebaseUI-Android,SUPERCILEX/FirebaseUI-Android,JosefHruska/FirebaseUI-Android,SUPERCILEX/FirebaseUI-Android,SUPERCILEX/FirebaseUI-Android,samtstern/FirebaseUI-Android,firebase/FirebaseUI-Android,samtstern/FirebaseUI-Android,firebase/FirebaseUI-Android