Update custom-directive.md (vuejs#1263)

Author: fishmandev

src/guide/custom-directive.md

@@ -214,4 +214,4 @@ app.component('my-component', {
 
 Unlike attributes, directives can't be passed to a different element with `v-bind="$attrs"`.
 
-With [fragments](/guide/migration/fragments.html#overview) support, components can potentially have more than one root nodes. When applied to a multi-root component, directive will be ignored and the warning will be thrown.
+With [fragments](/guide/migration/fragments.html#overview) support, components can potentially have more than one root node. When applied to a multi-root component, directive will be ignored and the warning will be thrown.