fix: use X | None syntax instead of Optional in py310 code structure example

Author: martinhewing

docs_src/tutorial/code_structure/tutorial002_py310/hero_model.py

@@ -1,4 +1,4 @@
-from typing import TYPE_CHECKING, Optional
+from typing import TYPE_CHECKING
 
 from sqlmodel import Field, Relationship, SQLModel
 
@@ -13,4 +13,4 @@ class Hero(SQLModel, table=True):
     age: int | None = Field(default=None, index=True)
 
     team_id: int | None = Field(default=None, foreign_key="team.id")
-    team: Optional["Team"] = Relationship(back_populates="heroes")
+    team: "Team | None" = Relationship(back_populates="heroes")