updated markdown syntax for better formatting

Author: Taylor Savage

firebase-collection.html

@@ -12,7 +12,8 @@
 <link rel="import" href="firebase-query-behavior.html">
 
 <!--
-**Note: This element is for the older Firebase 2 API**
+* **Note: This element is for the older Firebase 2 API**
+
 For the latest official Firebase 3.0-compatible component from the Firebase team,
 see the [polymerfire](https://github.com/firebase/polymerfire) component.
 In this new component, the `firebase-collection` element has been replaced