fix Stream#flatten example

```
Welcome to Scala version 2.10.3 (Java HotSpot(TM) 64-Bit Server VM, Java 1.7.0_45).
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scala> val sov: Stream[Vector[Int]] = Vector(0) #:: Vector(0, 0) #:: sov.zip(sov.tail).map { n => n._1 ++ n._2 }
sov: Stream[Vector[Int]] = Stream(Vector(0), ?)

scala> sov flatten take 10 mkString ", "
<console>:1: error: ';' expected but integer literal found.
       sov flatten take 10 mkString ", "
                        ^

scala> sov.flatten take 10 mkString ", "
res0: String = 0, 0, 0, 0, 0, 0, 0, 0, 0, 0
```