Update getting_started_browser.md

found that if you include blanket at the top, it fails without error message (even with the debug flag on) in both FF and Chrome. by comparing to http://blanketjs.org/examples/backbone-koans-qunit/index.html?coverage=true step-by-step, i found out what i was doing wrong. maybe worth mentioning in the instructions in case more people maybe the same noob error :)