[cairo] Update MacPorts instructions on download page

[Ryan Schmidt]

On this page:

http://cairographics.org/download/

This passage should be deleted:


<p>If you want to use the Quartz or ATSUI backends, you would do: </p>

<pre><code>   sudo port install cairo +atsui +quartz
</code></pre>


There are no +atsui and +quartz variants anymore since as of cairo  
1.6.x Quartz support is considered stable and is on by default.


Also, the P in all occurrences of "MacPorts" should be capitalized.