docs: Fix the incorrect code in the openai-chat.adoc document (#3493)

Signed-off-by: aisibo1 <aisibo1@jd.com>
(cherry picked from commit e9646c2)

Author: ai-afk

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

@@ -632,7 +632,7 @@ OpenAiApi openAiApi = OpenAiApi.builder()
     .build();
 
 // Create a chat model with the custom OpenAiApi instance
-OpenAiChatmodel chatModel = OpenAiChatModel.builder()
+OpenAiChatModel chatModel = OpenAiChatModel.builder()
     .openAiApi(openAiApi)
     .build();
 // Build the ChatClient using the custom chat model