Fix typo in ChatClient docs

- correct response key from "generation" to "completion" in example code

The example response in the documentation incorrectly uses "generation" as the response key. This commit updates it to "completion" to accurately reflect the API's actual response format.

Author: LuizyHub

spring-ai-docs/src/main/antora/modules/ROOT/pages/api/chatclient.adoc

@@ -234,7 +234,7 @@ When calling the application endpoint via curl, the result is:
 [source,bash]
 ----
 ❯ curl localhost:8080/ai/simple
-{"generation":"Why did the pirate go to the comedy club? To hear some arrr-rated jokes! Arrr, matey!"}
+{"completion":"Why did the pirate go to the comedy club? To hear some arrr-rated jokes! Arrr, matey!"}
 ----
 
 === Default System Text with parameters