map() example requires magrittr 1.5

The last example in `map` does not work with magrittr 1.0.1. It throws an error at

```r
mtcars %>%
  split(.$cyl)
```

You either need `%>%` from magrittr 1.5 or do

```r
mtcars %>%
  split(mtcars$cyl)
```