Fix a typo in polymorphism.md (#2562)
diff --git a/docs/polymorphism.md b/docs/polymorphism.md
index b7ea31f..2661e04 100644
--- a/docs/polymorphism.md
+++ b/docs/polymorphism.md
@@ -542,7 +542,7 @@
> You can get the full code [here](../guide/example/example-poly-13.kt).
-However, the `Any` is a class and it is not serializable:
+However, `Any` is a class and it is not serializable:
```text
Exception in thread "main" kotlinx.serialization.SerializationException: Serializer for class 'Any' is not found.