README.md: Update link to video.js plugin docs

This patch updates the URL of the link to video.js' plugins
documentation, which was moved from docs/plugins.md to
docs/guides/plugins.md in this commit:

    commit f318eb9f1fe5b17f37dac1cb8762f7d01400d210
    Author: Steve Heffernan <steve@zencoder.com>
    Date:   Mon Oct 28 18:25:28 2013 -0700

        Close GH-801: API Doc Generation 2. Fixes #747

See: https://github.com/videojs/video.js/commit/f318eb9f1fe5b17f37dac1cb8762f7d01400d210