fix css <link> tag in index demo

Fixes an incorrect demo code in index:

From:
 `<link rel="bower_components/angular-tour/dist/angular-tour.css" /> `

To:
 `<link href="bower_components/angular-tour/dist/angular-tour.css" rel="stylesheet"/> `

A trivial typo but might trick someone copy/pasting from there and then wondering why the popups don't get the proper styling.