update v-model.md (vuejs#1259)

Author: wxsms

src/guide/migration/v-model.md

@@ -86,7 +86,7 @@ Then the parent could listen to that event and update a local data property, if
 <ChildComponent :title="pageTitle" @update:title="pageTitle = $event" />
 ```
 
-For convenience, we had a shorthand for this pattern with the .sync modifier:
+For convenience, we had a shorthand for this pattern with the `.sync` modifier:
 
 ```html
 <ChildComponent :title.sync="pageTitle" />