doc: improved multi-root component custom directive warning's clarity

Author: wenfangdu

src/guide/migration/custom-directives.md

@@ -29,7 +29,7 @@ Here’s an example of this:
 Vue.directive('highlight', {
   bind(el, binding, vnode) {
     el.style.background = binding.value
-  }
+  },
 })
 ```
 
@@ -57,7 +57,7 @@ const MyDirective = {
   beforeUpdate() {}, // new
   updated() {},
   beforeUnmount() {}, // new
-  unmounted() {}
+  unmounted() {},
 }
 ```
 
@@ -73,7 +73,7 @@ const app = Vue.createApp({})
 app.directive('highlight', {
   beforeMount(el, binding, vnode) {
     el.style.background = binding.value
-  }
+  },
 })
 ```
 
@@ -100,5 +100,5 @@ mounted(el, binding, vnode) {
 ```
 
 :::warning
-With [fragments](/guide/migration/fragments.html#overview) support, components can potentially have more than one root node. When applied to a multi-root component, a directive will be ignored and a warning will be logged.
+With [fragments](/guide/migration/fragments.html#overview) support, components can potentially have more than one root node. When applied to a multi-root component, a custom directive will be ignored and a warning will be logged.
 :::