Fix syntax error on given example - Add comma (#1758)

Author: emazyka

src/v2/guide/components-registration.md

@@ -86,7 +86,7 @@ Then define the components you'd like to use in a `components` option:
 
 ```js
 new Vue({
-  el: '#app'
+  el: '#app',
   components: {
     'component-a': ComponentA,
     'component-b': ComponentB