update changelog for #1535 (#1553)

Signed-off-by: Craig Disselkoen <[email protected]>

Author: cdisselkoen

cedar-policy/CHANGELOG.md

@@ -18,7 +18,7 @@ Cedar Language Version: TBD
 - Changed `Entities::add_entities` and `Entities::from_entities` to ignore structurally equal entities with the same Entity UID.
 - For `protobufs` experimental feature, a number of changes to the interface and
   the Protobuf format definitions, as we continue to iterate towards making this
-  feature stable. (#1488, #1495, #1506)
+  feature stable. (#1488, #1495, #1506, #1535)
 - `datetime` is now a default feature (#1541)
 - Bump MSRV to 1.81 (#1542)