Typo on slide #19 and #21 of 'Features'

There are a few typos in chapter 9 'Features' on slides #19 and #21. The code references columns `[["1_pos"]]`, `[["2_pos"]]` and `[["3_pos"]]` of the `dt` dataframe. However those columns are actually named `[["pos_1"]]`, `[["pos_2"]]` and `[["pos_3"]]`. 