Fix README: use `matrix` insted of `array`

I fix sample code on README.md, which doesn't work on julia 0.2.0 with DataArrays 0.0.3.
I'm afraid that it might be DataArrays's bug, using `matrix` instead of `array` seems to be fine.

```julia
array(iris[:, 1:4])'

#=>  array not defined
```