Fix typo

Author: ttskch

testing.rst

@@ -757,7 +757,7 @@ their type::
     $form['photo']->upload('/path/to/lucas.jpg');
 
     // In the case of a multiple file upload
-    $form['my_form[field][O]']->upload('/path/to/lucas.jpg');
+    $form['my_form[field][0]']->upload('/path/to/lucas.jpg');
     $form['my_form[field][1]']->upload('/path/to/lisa.jpg');
 
 .. tip::