docs: improve sample code

Author: kenjis

user_guide_src/source/libraries/curlrequest/028.php

@@ -1,6 +1,8 @@
 <?php
 
-$post_data = [
-    'foo'      => 'bar',
-    'userfile' => new \CURLFile('/path/to/file.txt'),
-];
+$client->request('POST', '/post', [
+    'multipart' => [
+        'foo'      => 'bar',
+        'userfile' => new \CURLFile('/path/to/file.txt'),
+    ],
+]);