Update iron-icons.html

[David-Mulder]

Fix links in documentation, at the very least they don't work in the element catalog and neither my 'fix' nor the current links work when you open the page in bower_components (bower_components links require ../iron-icon, but I think the Element Catalog is more often used)

[notwaldorf]

I think you're right! We did a similar fix recently for paper-tabs as well. Thanks!