docs: adjust additional properties example (#302)

Author: stainless-app[bot]

README.md

@@ -211,7 +211,7 @@ if (field.isMissing()) {
 Sometimes, the server response may include additional properties that are not yet available in this library's types. You can access them using the model's `_additionalProperties` method:
 
 ```java
-JsonValue secret = card._additionalProperties().get("secret_field");
+JsonValue secret = address._additionalProperties().get("secret_field");
 ```
 
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