PEP 752: minor stylistic update (#4332)

* PEP 752: minor stylistic update

* add link to the reference implementation

Author: ofek

peps/pep-0752.rst

@@ -382,12 +382,13 @@ installation.
 Reference Implementation
 ========================
 
-None at this time.
+A complete reference implementation of this PEP is available in
+`PR #17691 <https://github.com/pypi/warehouse/pull/17691>`__.
 
 Rejected Ideas
 ==============
 
-Granting reservations to users
+Granting Reservations to Users
 ------------------------------
 
 As package repositories have a flat namespace, allowing any user to reserve a