docs: Fix "Use of stdlib dataclasses with BaseModel" bug (#8491)

Co-authored-by: Sydney Runkle <54324534+sydney-runkle@users.noreply.github.com>

docs/concepts/dataclasses.md

@@ -250,7 +250,7 @@ import dataclasses
 from datetime import datetime
 from typing import Optional
 
-from pydantic import BaseModel, ValidationError
+from pydantic import BaseModel, ConfigDict, ValidationError
 
 
 @dataclasses.dataclass(frozen=True)
@@ -265,6 +265,9 @@ class File:
 
 
 class Foo(BaseModel):
+    # Required so that pydantic revalidates the model attributes
+    model_config = ConfigDict(revalidate_instances='always')
+
     file: File
     user: Optional[User] = None