Change Mac OS X reference to macOS

Mac OS X changed its name to OS X in 2011 and then to macOS in 2016.

source/jp2a/index.html

@@ -94,7 +94,7 @@ <h2>If you need to build jpeglib yourself</h2>
 </pre>
 </div>
 
-<h2>Mac OS X</h2>
+<h2>macOS</h2>
 <div class="text">
 jp2a is part of <a href="https://www.macports.org">MacPorts</a>, so if you do a