docs(firestore-bigquery-export): clarify BigQuery billing (#106)

Author: jhuleatt

firestore-bigquery-export/PREINSTALL.md

@@ -26,8 +26,8 @@ Learn more about using this script to [backfill your existing collection](https:
 
 This extension uses other Firebase or Google Cloud Platform services which may have associated charges:
 
++   BigQuery (this extension writes to BigQuery with [streaming inserts](https://cloud.google.com/bigquery/pricing#streaming_pricing))
 +   Cloud Firestore
-+   BigQuery
 +   Cloud Functions
 
 When you use Firebase Extensions, you're only charged for the underlying resources that you use. A paid-tier billing plan is only required if the extension uses a service that requires a paid-tier plan, for example calling to a Google Cloud Platform API or making outbound network requests to non-Google services. All Firebase services offer a free tier of usage. [Learn more about Firebase billing.](https://firebase.google.com/pricing)