Example exhibits integer overflow/incorrect result silently

In https://github.com/scala/scala.github.com/blob/master/overviews/parallel-collections/overview.md, under "Some Examples", the fold/sum example overflows.

One might choose a smaller range, or point out the gotcha for the unwary.

```
scala> (1 to 1e6.toInt).map(_.toLong).sum
res42: Long = 500000500000
```
