Fix sample code in migration85.new-features

Author: mowangjuanzi

appendices/migration85/new-features.xml

@@ -1,5 +1,5 @@
 <?xml version="1.0" encoding="utf-8"?>
-<!-- EN-Revision: ec45af749649dc0d6a23eaedeed1b601f7460813 Maintainer: mowangjuanzi Status: ready -->
+<!-- EN-Revision: ce397343296708654c8cdac774e23a9ee47ab27d Maintainer: mowangjuanzi Status: ready -->
 <sect1 xml:id="migration85.new-features" xmlns:xlink="http://www.w3.org/1999/xlink">
  <title>新功能</title>
 
@@ -65,7 +65,7 @@ print $result . PHP_EOL;  // Prints "11"
 
 #[\NoDiscard]
 function concat(string $a, string $b): string {
-     return a + b;
+     return a . b;
 }
 
 // concat() 的返回值应要么被使用，要么通过在 xxx.php 中将其强制转换为 (void) 来有意忽略。