fix: typo MacOS to macOS

Co-authored-by: USAMI Kenta <[email protected]>

Author: KentarouTakeda

appendices/migration84/new-classes.xml

@@ -73,7 +73,7 @@
   <title>PCNTL</title>
   <simplelist>
    <!-- Should this use <enumname> ? -->
-   <member><classname>Pcntl\QosClass</classname> (MacOSのみ)</member>
+   <member><classname>Pcntl\QosClass</classname> (macOSのみ)</member>
   </simplelist>
  </sect2>