correcting indexOf condition in History.getFragment

https://github.com/jashkenas/backbone/blob/master/backbone.js#L1350

`if (!fragment.indexOf(root))` — this will be true if root will be on the beginning of the fragment, because indexOf returns `-1` if it doesn't find the argument string according to https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/indexOf and if I understand correctly, we need to find out if root is not in the fragment, which will be `if (fragment.indexOf(root) < 0)`