Fix typo in dom-repeat.html

"should me made" -> "should be made"

lib/elements/dom-repeat.html

@@ -67,7 +67,7 @@
    * Notifications for changes to items sub-properties will be forwarded to template
    * instances, which will update via the normal structured data notification system.
    *
-   * Mutations to the `items` array itself should me made using the Array
+   * Mutations to the `items` array itself should be made using the Array
    * mutation API's on `Polymer.Base` (`push`, `pop`, `splice`, `shift`,
    * `unshift`), and template instances will be kept in sync with the data in the
    * array.