[Docs] Document using a custom RG (#247)

* Document using a custom RG

* Update docs/guides/OPENAI_CHAT_COMPLETION.md

---------

Co-authored-by: Alexander Dümont <[email protected]>

Author: MatKuhr

docs/guides/OPENAI_CHAT_COMPLETION.md

@@ -47,6 +47,7 @@ In addition to the prerequisites above, we assume you have already set up the fo
     - Refer
       to [How to deploy a model to AI Core](https://help.sap.com/docs/sap-ai-core/sap-ai-core-service-guide/create-deployment-for-generative-ai-model-in-sap-ai-core)
       for setup instructions.
+    - In case the model is deployed in a custom resource group, refer to [this section](#using-a-custom-resource-group).
     - <details>
       <summary>Example deployed model from the AI Core <code>/deployments</code> endpoint</summary>
 
@@ -97,6 +98,15 @@ OpenAiChatCompletionOutput result =
 String resultMessage = result.getContent();
 ```
 
+### Using a Custom Resource Group
+
+```java
+var destination = new AiCoreService()
+    .getInferenceDestination("custom-rg")
+    .forModel(GPT_4O);
+OpenAiClient.withCustomDestination(destination);
+```
+
 ### Message history
 
 ```java

foundation-models/openai/src/main/java/com/sap/ai/sdk/foundationmodels/openai/OpenAiClient.java

@@ -87,7 +87,7 @@ public OpenAiClient withApiVersion(@Nonnull final String apiVersion) {
    *
    * <pre>{@code
    * var destination = new AiCoreService().getInferenceDestination("custom-rg").forModel(GPT_4O);
-   * OpenAiClient.withCustomDestination(destination).withApiVersion("2024-10-21");
+   * OpenAiClient.withCustomDestination(destination);
    * }</pre>
    *
    * @param destination The specific {@link HttpDestination} to use.