Update FAQ.md

> ### Changes
> Fix typo `useRefreshTokenFallback` to `useRefreshTokensFallback` in FAQ.md 
> 
> ### Checklist
> * [x]  I have read the [Auth0 general contribution guidelines](https://github.com/auth0/open-source-template/blob/master/GENERAL-CONTRIBUTING.md)
> * [x]  I have read the [Auth0 Code of Conduct](https://github.com/auth0/open-source-template/blob/master/CODE-OF-CONDUCT.md)
> * [x]  All code quality tools/guidelines have been run/followed

FAQ.md

@@ -85,7 +85,7 @@ v1 of the SDK used an iframe as a backup if no refresh token was available. You
 To mitigate this, use one of the following solutions:
 
 - Handle the error, and log the user in again using `loginWithRedirect` or `loginWithPopup`.
-- Revert to the v1 behaviour by setting `useRefreshTokenFallback` to `true`.
+- Revert to the v1 behaviour by setting `useRefreshTokensFallback` to `true`.
 
 In all cases, the fallback will not work in browsers where third-party cookies are blocked, unless you are using [custom domains](https://auth0.com/docs/customize/custom-domains).