Update index.jade, fixed namespace error

I was just beginning to learn using MongoDB via Mongoose, using your tutorial at http://mongoosejs.com/docs/index.html
Unfortunately the "getting started" tutorial has a severe flaw which prevents any commits from succeeding and almost drove me off to mongojs. You write:
`var Kitten = mongoose.model('Kitten', kittySchema)`
whereas according to http://stackoverflow.com/questions/10156623/mongoose-js-instance-save-callback-not-firing you should be writing:
`var Kitten = db.model('Kitten', kittySchema)`
lest the  model will be saved to the wrong namespace. I tested both versions in a coffeescript adaption of the code.