Change link caption of cebe/markdown

I choose this name instead of just "Markdown" to make it more distinguishable.

Thanks fore mentioning it here, btw :)

README.md

 * [Dflydev Markdown](https://github.com/dflydev/dflydev-markdown) - Another Markdown parser.
 * [Parsedown](https://github.com/erusev/parsedown) - Another Markdown parser.
 * [Ciconia](https://github.com/kzykhys/Ciconia) - Another Markdown parser that supports Github flavoured Markdown.
-* [Markdown](https://github.com/cebe/markdown) - An extensible Markdown parser.
+* [cebe/markdown](https://github.com/cebe/markdown) - An extensible and fast Markdown parser.
 * [HTML5 PHP](https://github.com/Masterminds/html5-php) - An HTML5 parser and serializer library.
 
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