fix: add chat in the function to fix 'No overload matches this call.

docs/pages/docs/guides/providers/openai.mdx

@@ -138,7 +138,7 @@ export async function POST(req: Request) {
   const { prompt } = await req.json();
 
   // Ask OpenAI for a streaming completion given the prompt
-  const response = await openai.completions.create({
+  const response = await openai.chat.completions.create({
     model: 'gpt-3.5-turbo-instruct',
     max_tokens: 2000,
     stream: true,