Switch CDN from "unpkg.com" to "cdn.jsdelivr.net"

In Febrary 2020 ([#1022](https://github.com/docsifyjs/docsify/pull/1022)), docsify has switch [its CDN](https://docsify.js.org/#/cdn) from `unpkg.com` to `cdn.jsdelivr.net`. 

As a good plugin to use, it would be nice if *docsify-toc* can be hosted on `cdn.jsdelivr.net` ; more coherent with other docsify plugins.