fix get command from type 'new_chat_members'

Author: Tsl

src/Telegram.php

@@ -440,7 +440,7 @@ public function processUpdate(Update $update)
                 'left_chat_member',
                 'migrate_from_chat_id',
                 'migrate_to_chat_id',
-                'new_chat_member',
+                'new_chat_members',
                 'new_chat_photo',
                 'new_chat_title',
                 'pinned_message',