Fixed array key typo on possible $_FILES structure in PSR-7

Author: jmossetchafik

accepted/PSR-7-http-message.md

@@ -535,7 +535,7 @@ array (
                 'avatars' => array (
                     0 => 90996,
                     1 => 90996,
-                    3 => 90996,
+                    2 => 90996,
                 ),
             ),
         ),