Fix version-added directive for polymorphic_serialization (pydantic#12910)

Author: rmorshea

docs/concepts/serialization.md

@@ -609,7 +609,7 @@ print(m.model_dump())  # (1)!
 
 ### Polymorphic serialization
 
-/// version-added | v2.12
+/// version-added | v2.13
 Polymorphic serialization was added as an better alternative to the [serialize as any](#serializing-as-any) behavior, and only
 applies to Pydantic models and Pydantic dataclasses.
 ///