Fix typo

src/lib/template/dom-repeat.html

@@ -51,7 +51,7 @@
 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.