docs: remove broken link (fixed #22)

Author: serkodev

docs/guide/plugins.md

@@ -33,7 +33,6 @@ Thanks to the [markdown-it](https://github.com/markdown-it) community has develo
 - [markdown-it-container](https://github.com/markdown-it/markdown-it-container)
 - [markdown-it-ins](https://github.com/markdown-it/markdown-it-ins)
 - [markdown-it-mark](https://github.com/markdown-it/markdown-it-mark)
-- [markdown-it-kbd](https://github.com/markdown-it/markdown-it-kbd)
 
 Browse all markdown-it plugins on the [npm](https://www.npmjs.org/browse/keyword/markdown-it-plugin).