Update how-jquery-works.md

I'm a complete newbie here (i.e. your audience), so I although I guessed I was supposed to put the jQuery snippet somewhere, I wasn't clear where. Was I supposed to put it in the jquery.js which I had just downloaded? Or in the HTML file somewhere? I wasn't sure. Eventually I figured out it belonged in the HTML file where it was written "// Your code goes here' (and felt stupid for not realising that). Anyway, I recommend making these instructions even more explicit.