Fix property hooks example (#4550)

Property hooks are incompatible with readonly properties

Author: BackEndTea

language/oop5/property-hooks.xml

@@ -315,7 +315,7 @@ class Example
    <programlisting role="php">
 <![CDATA[
 <?php
-readonly class Rectangle
+class Rectangle
 {
     // A virtual property.
     public int $area {