Simplify Stringable example with PHP 8 syntax (#3514)

Stringable is introduced in PHP 8.0, so constructor
promotion can be assumed.

language/predefined/stringable.xml

@@ -52,26 +52,20 @@
 <![CDATA[
 <?php
 class IPv4Address implements Stringable {
-    private string $oct1;
-    private string $oct2;
-    private string $oct3;
-    private string $oct4;
-
-    public function __construct(string $oct1, string $oct2, string $oct3, string $oct4) {
-        $this->oct1 = $oct1;
-        $this->oct2 = $oct2;
-        $this->oct3 = $oct3;
-        $this->oct4 = $oct4;
-    }
+    public function __construct(
+        private string $oct1,
+        private string $oct2,
+        private string $oct3,
+        private string $oct4,
+    ) {}
 
     public function __toString(): string {
         return "$this->oct1.$this->oct2.$this->oct3.$this->oct4";
     }
 }
 
 function showStuff(string|Stringable $value) {
-    // A Stringable will get converted to a string here by calling
-    // __toString.
+    // For a Stringable, this will implicitliy call __toString().
     print $value;
 }