Update custom-directives.md

The author forgot to change directive's name from the previous example. In the previous example the name of directive was "highlight" but in this example it is "focus"

src/guide/reusability/custom-directives.md

@@ -142,7 +142,7 @@ const focus = {
 
 export default {
   directives: {
-    // включение v-highlight в шаблоне
+    // включение v-focus в шаблоне
     focus
   }
 }