docs: update stream text recipe message type (#4861)

content/cookbook/01-next/21-stream-text-with-chat-prompt.mdx

@@ -62,14 +62,14 @@ export default function Page() {
 Next, let's create the `/api/chat` endpoint that generates the assistant's response based on the conversation history.
 
 ```typescript filename='app/api/chat/route.ts'
-import { CoreMessage, streamText } from 'ai';
+import { streamText, UIMessage } from 'ai';
 import { openai } from '@ai-sdk/openai';
 
 export async function POST(req: Request) {
-  const { messages }: { messages: CoreMessage[] } = await req.json();
+  const { messages }: { messages: UIMessage[] } = await req.json();
 
   const result = streamText({
-    model: openai('gpt-4'),
+    model: openai('gpt-4o'),
     system: 'You are a helpful assistant.',
     messages,
   });