Update README.md to correct error in example for zombie initialization

No zombie.Browser() object in zombie anymore..

Using the example code results in an error:
<pre>
this.browser = new zombie.Browser(); // this.browser will be available in st
                 ^
TypeError: undefined is not a function
</pre>

So either do this (done in my patch):
```javascript
  this.browser = new zombie()
```

Or do this (per the zombie examples):
```javascript
     var Browser = require('zombie');
```
...
```javascript
    this.browser = new Browser();
```