Broken Link Fix

Fixed a broken link in "Add properties and methods" section.

"When you’re creating a new element, you’ll often need to expose a *public API* so users can configure it." 

Current link is: [public API](/polymer/polymer.html#published-properties) >> http://www.polymer-project.org/polymer/polymer.html#published-properties
Correct link is: [public API](docs/polymer/polymer.html#published-properties) >> http://www.polymer-project.org/docs/polymer/polymer.html#published-properties