PEP 747: fix rst syntax (#4004)

peps/pep-0747.rst

@@ -223,7 +223,7 @@ For example, if a static type checker encounters the expression ``str | None``,
 it may normally evaluate its type as ``UnionType`` because it produces a
 runtime value that is an instance of ``types.UnionType``. However, because
 this expression is a valid type expression, it is also assignable to the
-type ``TypeForm[str | None]``:
+type ``TypeForm[str | None]``::
 
   v1_actual: UnionType = str | None  # OK
   v1_type_form: TypeForm[str | None] = str | None  # OK