Change readme exception example

I would change this:

``` javascript
expect(fn).to.throwException(function (e) { // get the exception object
expect(e).to.be.a(SyntaxError);
});
```

to

``` javascript
expect(fn).to.throwException(function (e) { // get the exception object
expect(e.constructor).to.be(SyntaxError);
});
```

to ignore inheritance. It surely depends on the test, but I think that you usually want to test for a specific exception, not a "kind of" exception.

Just a suggestion ... :)