Update league/commonmark description

This change better aligns the description shown here with the project's description

README.md

 *Libraries for working with markup.*
 
 * [Cebe Markdown](https://github.com/cebe/markdown) - An fast and extensible Markdown parser.
-* [CommonMark PHP](https://github.com/thephpleague/commonmark) - A Markdown parser which supports the full [CommonMark spec](https://spec.commonmark.org/).
+* [CommonMark PHP](https://github.com/thephpleague/commonmark) - Highly-extensible Markdown parser which fully supports the [CommonMark spec](https://spec.commonmark.org/).
 * [Decoda](https://github.com/milesj/decoda) - A lightweight markup parser library.
 * [Emoji](https://github.com/heyupdate/Emoji) - A library that converts unicode characters and names into emoji images.
 * [HTML to Markdown](https://github.com/thephpleague/html-to-markdown) - Converts HTML into Markdown.