Update installation instructions for macOS on web site

The installation section of your [web site](https://csl.name/jp2a/) shows this information:

> ### Mac OS X
>
> jp2a is now part of [DarwinPorts](http://www.darwinports.org/), so if you do a
> ```
> sudo port install jp2a
> ```
> then everything should be installed.

These names are outdated:

* Mac OS X is now known as macOS
* DarwinPorts is now known as MacPorts

I suggest the above passage should be changed to look like this:

> ### macOS
>
> You can use [MacPorts](https://ports.macports.org/port/jp2a):
> ```
> sudo port install jp2a

This wording fits better with the existing wording for other operating systems.

Note that I'm suggesting changing the link for MacPorts to point our new jp2a port info page instead of to the MacPorts homepage.

Find the website's installation section containing the quoted “Mac OS X” and “DarwinPorts” passage. Update the heading, MacPorts link, and installation wording as proposed, then verify the rendered page shows the current names and link.

Written by the indexing model from the issue text.