Replace image 'url' with 'destination' in DTD

[ph1ll]

The DTD defines a 'url' attribute for an image, however both cmark and CommonMark.js appear to use 'destination' instead (which is also consistent with a link).

http://spec.commonmark.org/dingus/

![foo](/url "title")

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE document SYSTEM "CommonMark.dtd">

<document xmlns="http://commonmark.org/xml/1.0">
  <paragraph>
    <image destination="/url" title="title">
      <text>foo</text>
    </image>
  </paragraph>
</document>

This MR updates the DTD to use 'destination' instead of 'url'. I couldn't see any other references in the spec indicating which attribute is actually correct, but if it is the case that cmark/commonmark.js are actually incorrect and not following the DTD, please close this.

[jgm]

Better late than never!