interfaces.xml Make the class property truly virtual (#4152)

* interfaces.xml Fix the example error

In the footsteps of the issue: #4151

* Update interfaces.xml minimize spaces

Author: mmalferov

language/oop5/interfaces.xml

@@ -154,7 +154,9 @@ class C2 implements I
     // This example creates a virtual property, which is fine.
     public string $writeable {
         get => $this->written;
-        set => $value;
+        set {
+            $this->written = $value;
+        }
     }
 
     // This property requires both read and write be possible,