fix: classes renaming to correspond to UML diagram

Behavioral/State/OrderDone.php

@@ -0,0 +1,18 @@
+<?php
+
+declare(strict_types=1);
+
+namespace DesignPatterns\Behavioral\State;
+
+class OrderDone implements StateOrder
+{
+    public function proceedToNext(ContextOrder $context)
+    {
+        // there is nothing more to do
+    }
+
+    public function toString(): string
+    {
+        return 'done';
+    }
+}