Fix overescaping in <paper-item> docs

http://www.polymer-project.org/docs/elements/paper-elements.html#paper-item currently says "To use as a link, put an &lt;a&gt; element in the item." I'm guessing this change will fix that. Hopefully it won't break anything else.
Merge pull request #4 from jyasskin/patch-1

fix overescaping in docs