docs: Fix tanstack responseMiddleware (#2489)

docs/docs/guides/storefront/connect-api/index.mdx

@@ -536,7 +536,7 @@ const requestMiddleware: RequestMiddleware = async (request) => {
 
 // Check all responses for a new session token
 const responseMiddleware: ResponseMiddleware = (response) => {
-    if (!(response instanceof Error) && response.errors) {
+    if (!(response instanceof Error) && !response.errors) {
         const authHeader = response.headers.get('vendure-auth-token');
         if (authHeader) {
             // If the session token has been returned by the Vendure