Fix import in doctest example (#14067)

Author: robert-s-lee

src/lightning_app/structures/dict.py

@@ -22,7 +22,7 @@ def __init__(self, **kwargs: T):
         .. doctest::
 
             >>> from lightning_app import LightningFlow, LightningWork
-            >>> from lightning_app.core import Dict
+            >>> from lightning_app.structures import Dict
             >>> class CounterWork(LightningWork):
             ...     def __init__(self):
             ...         super().__init__()

src/lightning_app/structures/list.py

@@ -24,7 +24,7 @@ def __init__(self, *items: T):
         .. doctest::
 
             >>> from lightning_app import LightningFlow, LightningWork
-            >>> from lightning_app.core import List
+            >>> from lightning_app.structures import List
             >>> class CounterWork(LightningWork):
             ...     def __init__(self):
             ...         super().__init__()