Update mismatch in main index.d.ts

[hsubox76]

I think this got overlooked in an update to database-types (https://github.com/firebase/firebase-js-sdk/blob/master/packages/database-types/index.d.ts#L88)

See #4250.

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No changes between base commit (487f8e1) and head commit (7b78351).

Test Logs

• Base (487f8e1d): https://github.com/firebase/firebase-js-sdk/actions/runs/452487575
• Head (7b783510): https://github.com/firebase/firebase-js-sdk/actions/runs/463997180