wrong output in section 8.2.1

[the output](https://github.com/r-spark/the-r-in-spark/blob/68b42f8c2b8f78ed9ce68bded4ae6549d2a423d5/data.Rmd#L99) of the `spark_read_csv(sc, "data-csv/")` is wrong.

the right output is:

```
# Source: spark<datacsv> [?? x 2]
   x         y
   <chr> <int>
1 a         1
2 b         2
3 c         3
4 a         1
5 b         2
6 c         3
```
