Fix typo.

src/lib/template/dom-repeat.html

@@ -194,7 +194,7 @@
        * Defines an initial count of template instances to render after setting
        * the `items` array, before the next paint, and puts the `dom-repeat`
        * into "chunking mode".  The remaining items will be created and rendered
-       * incrementally at each animation frame therof until all instnaces have
+       * incrementally at each animation frame therof until all instances have
        * been rendered.
        */
       initialCount: {