Add missing provider definition

[sourabh32]

added missing provider definition

[thatfiredev]

@sourabh32 In our docs, the provider definition is actually done in step 1: https://firebase.google.com/docs/auth/web/google-signin#handle_the_sign-in_flow_with_the_firebase_sdk

The snippet you just edited is for step 5 - we assume readers have already done step 1 by the time they get there, which means the provider has already beed defined.

[thatfiredev]

Closing this for now. But let me know if you disagree with my previous comment