Remove {{}} in on-click attribute in example.

iron-collapse.html

@@ -14,7 +14,7 @@
 `iron-collapse` creates a collapsible block of content.  By default, the content
 will be collapsed.  Use `opened` or `toggle()` to show/hide the content.
 
-    <button on-click="{{toggle}}">toggle collapse</button>
+    <button on-click="toggle">toggle collapse</button>
 
     <iron-collapse id="collapse">
       <div>Content goes here...</div>