Fix env name error for postgresql database (#365)

Author: timewalker08

README.md

@@ -166,7 +166,7 @@ Follow these steps to quickly set up and run the ChatGPT Retrieval Plugin:
    export PG_PORT=<postgres_port>
    export PG_USER=<postgres_user>
    export PG_PASSWORD=<postgres_password>
-   export PG_DATABASE=<postgres_database>
+   export PG_DB=<postgres_database>
 
    # Elasticsearch
    export ELASTICSEARCH_URL=<elasticsearch_host_and_port> (either specify host or cloud_id)