PEP 728: Fix code block syntax

Author: Viicos

peps/pep-0728.rst

@@ -258,7 +258,7 @@ If ``closed`` is not provided, the behavior is inherited from the superclass.
 If the superclass is TypedDict itself or the superclass does not have ``closed=True``
 or the ``extra_items`` parameter, the previous TypedDict behavior is preserved:
 arbitrary extra items are allowed. If the superclass has ``closed=True``, the
-child class is also closed.
+child class is also closed::
 
     class BaseMovie(TypedDict, closed=True):
         name: str