docs: use 200 for routes response across docs (#14418)

www/apps/book/app/learn/fundamentals/api-routes/responses/page.mdx

@@ -58,7 +58,7 @@ export const GET = async (
   req: MedusaRequest,
   res: MedusaResponse
 ) => {
-  res.status(201).json({
+  res.status(200).json({
     message: "Hello, World!",
   })
 }