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

[axel3rd]

In Febrary 2020 (#1022), docsify has switch its CDN from unpkg.com to cdn.jsdelivr.net.

As a main plugin, it would be nice if docsify-edit-on-github can be hosted on cdn.jsdelivr.net ; more coherent with other docsify plugins.

[njleonzhang]

@axel3rd enabled，thx for suggestion.

https://cdn.jsdelivr.net/npm/docsify-edit-on-github