Update the signature of the teardown function (#3730)

Co-authored-by: David Lord <davidism@gmail.com>

docs/appcontext.rst

@@ -119,7 +119,7 @@ For example, you can manage a database connection using this pattern::
         return g.db
 
     @app.teardown_appcontext
-    def teardown_db():
+    def teardown_db(exception):
         db = g.pop('db', None)
 
         if db is not None: