PEP 728: Fix minor issues with examples (#4475)

Jukka pointed out the incorrect use of extra_items here:
  https://discuss.python.org/t/pep-728-typeddict-with-typed-extra-items/45443/157

Also fixed a minor issue with a class missing its body.

Author: PIG208

peps/pep-0728.rst

@@ -215,7 +215,7 @@ the ``extra_items`` argument::
 
 ``extra_items`` is inherited through subclassing::
 
-    class MovieBase(TypedDict, extra_items=int | None):
+    class MovieBase(TypedDict, extra_items=ReadOnly[int | None]):
         name: str
 
     class Movie(MovieBase):
@@ -380,6 +380,7 @@ unless it is declared to be ``ReadOnly`` in the superclass::
         pass
 
     class Child(Parent, extra_items=int): # Not OK. Like any other TypedDict item, extra_items's type cannot be changed
+        pass
 
 Second, ``extra_items=T`` effectively defines the value type of any unnamed
 items accepted to the TypedDict and marks them as non-required. Thus, the above