Adjust ClassVar example to use pre-PEP-526 syntax. (Issue #28388)

Doc/library/typing.rst

@@ -845,8 +845,8 @@ The module defines the following classes, functions and decorators:
    and should not be set on instances of that class. Usage::
 
       class Starship:
-          stats: ClassVar[Dict[str, int]] = {} # class variable
-          damage: int = 10                     # instance variable
+          stats = {}  # type: ClassVar[Dict[str, int]] # class variable
+          damage = 10 # type: int                      # instance variable
 
    :data:`ClassVar` accepts only types and cannot be further subscribed.