Fix typo in browser.js comments

https://github.com/hueniverse/hawk/issues/111

https://github.com/hueniverse/hawk/blob/master/lib/browser.js#L146

```
    /*
        request:    object created via 'new XMLHttpRequest()' after response received
        artifacts:  object recieved from header().artifacts
        options: {
            payload:    optional payload received
            required:   specifies if a Server-Authorization header is required. Defaults to 'false'
        }
    */
```

s/recieved/received/