Docs : Add missing import in documentation example (#1016)

Co-authored-by: Jason Liu <[email protected]>

docs/examples/planning-tasks.md

@@ -20,7 +20,8 @@ With the OpenAI Function Call model, you can customize the planning process and
 Let's define the necessary Pydantic models to represent the query plan and the queries.
 
 ```python
-from typing import List
+import enum
+from typing import List, Literal
 from pydantic import Field, BaseModel