Fix property hooks exampleを取り込み

php/doc-en#4550

Author: siwa32

language/oop5/property-hooks.xml

@@ -316,7 +316,7 @@ class Example
    <programlisting role="php">
 <![CDATA[
 <?php
-readonly class Rectangle
+class Rectangle
 {
     // 仮想プロパティ
     public int $area {