Update Pydantic example in tutorial to Pydantic v2 syntax

This PR updates outdated code examples in body tutorial docs that still used Pydantic v1 patterns. Updated to Pydantic v2 syntax including:
- Replacing `.dict()` with `.model_dump()`
- Maintaining Python 3.10+ union types (`str | None`)

This ensures documentation is consistent with current FastAPI + Pydantic v2 behavior.

docs_src/body/tutorial002_py310.py

@@ -14,7 +14,7 @@ app = FastAPI()
 
 @app.post("/items/")
 async def create_item(item: Item):
-    item_dict = item.dict()
+    item_dict = item.model_dump()
     if item.tax is not None:
         price_with_tax = item.price + item.tax
         item_dict.update({"price_with_tax": price_with_tax})

docs_src/body/tutorial003_py310.py

@@ -14,4 +14,4 @@ app = FastAPI()
 
 @app.put("/items/{item_id}")
 async def update_item(item_id: int, item: Item):
-    return {"item_id": item_id, **item.dict()}
+    return {"item_id": item_id, **item.model_dump()}

docs_src/body/tutorial004_py310.py

@@ -14,7 +14,7 @@ app = FastAPI()
 
 @app.put("/items/{item_id}")
 async def update_item(item_id: int, item: Item, q: str | None = None):
-    result = {"item_id": item_id, **item.dict()}
+    result = {"item_id": item_id, **item.model_dump()}
     if q:
         result.update({"q": q})
     return result