PEP 728: Update the reference implementation section (#3722)

Signed-off-by: Zixuan James Li <[email protected]>

Author: PIG208

peps/pep-0728.rst

@@ -651,7 +651,7 @@ because such dict can be a subtype of dict::
     not_a_regular_dict: CustomDict = {"num": 1}
     int_dict: IntDict = not_a_regular_dict  # Not OK
 
-How to Teach this
+How to Teach This
 =================
 
 The choice of ``"__extra_items__"`` and the requirement of ``closed=True``
@@ -775,12 +775,10 @@ from the index signature so that it is possible to define a type like
 Reference Implementation
 ========================
 
-pyanalyze has `experimental support
-<https://github.com/quora/pyanalyze/blob/9bfc2c58467c87774a9950838402d2657b1486a0/pyanalyze/extensions.py#L590>`__
-for a similar feature.
+This proposal is supported in `pyright 1.1.352
+<https://github.com/microsoft/pyright/releases/tag/1.1.352>`_, and `pyanalyze
+0.12.0 <https://github.com/quora/pyanalyze/releases/tag/v0.12.0>`_.
 
-Reference implementation for this specific proposal, however, is not currently
-available.
 
 Acknowledgments
 ===============