Fix(typo): add missing closing bracket in docs

Author: linarkou

spring-ai-docs/src/main/antora/modules/ROOT/pages/api/retrieval-augmented-generation.adoc

@@ -26,7 +26,7 @@ Assuming you have already loaded data into a `VectorStore`, you can perform Retr
 ----
 ChatResponse response = ChatClient.builder(chatModel)
         .build().prompt()
-        .advisors(new QuestionAnswerAdvisor(vectorStore)
+        .advisors(new QuestionAnswerAdvisor(vectorStore))
         .user(userText)
         .call()
         .chatResponse();