fix(docs): use correct putAdditionalProperty value (#281)

README.md

@@ -158,9 +158,10 @@ Sometimes, the API may support other properties that are not yet supported in th
 you can attach them using the `putAdditionalProperty` method.
 
 ```java
+import com.lithic.api.models.core.JsonValue;
 CardCreateParams params = CardCreateParams.builder()
     // ... normal properties
-    .putAdditionalProperty("secret_param", "4242")
+    .putAdditionalProperty("secret_param", JsonValue.from("4242"))
     .build();
 ```