updated "reserves" link in "attributes" section (link was broken)

link was no longer correct due to changes in project structure(?), but still exists in "commit history":(https://github.com/Polymer/polymer/blob/d8d0a4d15d7fe9bbd69a0ab4b52c1ef52ec58920/src/declaration/attributes.js#L53) (this is link to source code which was actual when writing this doc). 

polymer-dev is now project containing "attributes.js file":(https://github.com/Polymer/polymer-dev/blob/fbae382197772afc123207ab8fc9379583ae50b0/src/declaration/attributes.js#L92) (by executing git blame I was able to find what was in attributes.file as the date of docs creation "2013-10-04":(https://github.com/Polymer/docs/blame/513ea8f0859767bf8a05a6826024388f0b81bf2a/docs/polymer/polymer.md#L28) )