Update README.md

Fixed missing import needed for os.getenv.

Author: flyingearl

docs/docs/06_sql_storage_sqlalchemy/04_configure_flask_sqlalchemy/README.md

@@ -58,6 +58,8 @@ def create_app():
 ## Add Flask-SQLAlchemy code to the app factory
 
 ```python title="app.py"
+import os
+
 from flask import Flask
 from flask_smorest import Api
 
@@ -107,4 +109,4 @@ We've done three things:
 
 :::tip How does SQLAlchemy know what tables to create?
 The line `import models` lets SQLAlchemy know what models exist in our application. Because they are `db.Model` instances, SQLAlchemy will look at their `__tablename__` and defined `db.Column` attributes to create the tables.
-:::
+:::