improve vue devtools referencing (#472)

Author: kazupon

src/api/index.md

@@ -779,7 +779,7 @@ All lifecycle hooks automatically have their `this` context bound to the instanc
 
   Allow the component to recursively invoke itself in its template. Note that when a component is registered globally with `Vue.component()`, the global ID is automatically set as its name.
 
-  Another benefit of specifying a `name` option is debugging. Named components result in more helpful warning messages. Also, when inspecting an app in the Vue devtool, unnamed components will show up as `<AnonymousComponent>`, which isn't very informative. By providing the `name` option, you will get a much more informative component tree.
+  Another benefit of specifying a `name` option is debugging. Named components result in more helpful warning messages. Also, when inspecting an app in the [vue-devtools](https://github.com/vuejs/vue-devtools), unnamed components will show up as `<AnonymousComponent>`, which isn't very informative. By providing the `name` option, you will get a much more informative component tree.
 
 ### extends