class exists bugfix (#39851)

Co-authored-by: Adam Rodriguez <adamr@healthendeavors.onmicrosoft.com>

src/Illuminate/Http/Client/Factory.php

@@ -123,7 +123,7 @@ public static function response($body = null, $status = 200, $headers = [])
 
         $response = new Psr7Response($status, $headers, $body);
 
-        return class_exists(GuzzleHttp\Promise\Create::class)
+        return class_exists(\GuzzleHttp\Promise\Create::class)
             ? \GuzzleHttp\Promise\Create::promiseFor($response)
             : \GuzzleHttp\Promise\promise_for($response);
     }