docs(firestore-bigquery-export) change heading level to h4 for script sections (#165)

Author: rachelsaunders

firestore-bigquery-export/PREINSTALL.md

@@ -16,13 +16,13 @@ Before installing this extension, you'll need to:
 +   [Set up Cloud Firestore in your Firebase project.](https://firebase.google.com/docs/firestore/quickstart)
 +   [Link your Firebase project to BigQuery.](https://support.google.com/firebase/answer/6318765)
 
-##### Backfill your BigQuery dataset
+#### Backfill your BigQuery dataset
 
 This extension only sends the content of documents that have been changed -- it does not export your full dataset of existing documents into BigQuery. So, to backfill your BigQuery dataset with all the documents in your collection, you can run the [import script](https://github.com/firebase/extensions/blob/master/firestore-bigquery-export/guides/IMPORT_EXISTING_DOCUMENTS.md) provided by this extension.
 
 **Important:** Run the import script over the entire collection _after_ installing this extension, otherwise all writes to your database during the import might be lost.
 
-##### Generate schema views
+#### Generate schema views
 
 After your data is in BigQuery, you can run the [schema-views script](https://github.com/firebase/extensions/blob/master/firestore-bigquery-export/guides/GENERATE_SCHEMA_VIEWS.md) (provided by this extension) to create views that make it easier to query relevant data. You only need to provide a JSON schema file that describes your data structure, and the schema-views script will create the views.