Update docs to use proper markdown for dartdocs.org

Hi @localvoid, I found your excellent library and intend to use it in a project of my own.

However, the docs at [dartdocs.org](http://www.dartdocs.org/documentation/diff_match_patch/0.2.0/index.html#diff_match_patch/diff_match_patch) are broken because the documentation comments are parsed as Markdown, and so the formatting is all broken (see diff() and match() for particularly bad examples). This PR fixes it, and makes (almost) no text content changes.

Let me know if there is anything you dislike. Thanks!