Replace custom element example link by the non-deprecated one (#805)

Author: ivangabriele

src/v2/guide/plugins.md

@@ -8,7 +8,7 @@ order: 18
 
 Plugins usually add global-level functionality to Vue. There is no strictly defined scope for a plugin - there are typically several types of plugins you can write:
 
-1. Add some global methods or properties. e.g. [vue-element](https://github.com/vuejs/vue-element)
+1. Add some global methods or properties. e.g. [vue-custom-element](https://github.com/karol-f/vue-custom-element)
 
 2. Add one or more global assets: directives/filters/transitions etc. e.g. [vue-touch](https://github.com/vuejs/vue-touch)