Chore: fix python model audit docs (#3632)

Author: georgesittas

docs/concepts/models/python_models.md

@@ -302,6 +302,7 @@ import typing as t
 from datetime import datetime
 
 import pandas as pd
+from sqlglot.expressions import to_column
 from sqlmesh import ExecutionContext, model
 
 @model(
@@ -317,7 +318,7 @@ from sqlmesh import ExecutionContext, model
         "name": "Name corresponding to the ID",
     },
     audits=[
-        ("not_null", {"columns": ["id"]}),
+        ("not_null", {"columns": [to_column("id")]}),
     ],
 )
 def execute(