PEP 728: Fix some small mistakes (#4554)

Noticed some small issues while adapting this PEP to the typing spec.

Author: JelleZijlstra

peps/pep-0728.rst

@@ -336,7 +336,7 @@ extra items::
         name: str
 
     def f(movie: Movie) -> None:
-        del movie["name"]  # Not OK. The value type of 'name' is 'Required[int]'
+        del movie["name"]  # Not OK. The value type of 'name' is 'Required[str]'
         del movie["year"]  # OK. The value type of 'year' is 'NotRequired[int]'
 
 Interaction with ``Unpack``
@@ -727,8 +727,8 @@ with signatures matching ``dict[str, VT]``
     reveal_type(not_required_num_dict.popitem())  # OK. Revealed type is 'tuple[str, int]'
 
     def f(not_required_num_dict: IntDictWithNum, key: str):
-      not_required_num_dict[key] = 42  # OK
-      del not_required_num_dict[key]  # OK
+        not_required_num_dict[key] = 42  # OK
+        del not_required_num_dict[key]  # OK
 
 :ref:`Notes on indexed accesses <pep728-type-narrowing>` from the previous section
 still apply.
@@ -742,7 +742,7 @@ because such dict can be a subtype of dict::
     def f(might_not_be_a_builtin_dict: dict[str, int]):
         int_dict: IntDict = might_not_be_a_builtin_dict # Not OK
 
-    not_a_builtin_dict: CustomDict = {"num": 1}
+    not_a_builtin_dict = CustomDict({"num": 1})
     f(not_a_builtin_dict)
 
 Runtime behavior