Fix compilation error in SearchBird example code

Following code in SearchBird example will produce compilation error:

```scala
value.split(" ").toSet foreach { token => ... }

error: missing parameter type for expanded function...
```

It should be:

```scala
value.split(" ").toSet[String] foreach { token => ... }
// or
value.split(" ").toSet foreach { token: String => ...}
```

Detailed discussion about the type inference can be found here:
http://stackoverflow.com/questions/5544536/type-inference-on-set-failing