Is sample_n described earlier? Use instead of slice_sample

[avahoffman]

          - [ ] Is `sample_n` described earlier? Use instead of `slice_sample`

Originally posted by @avahoffman in #148 (comment)

[carriewright11]

slice_sample() is introduced in the subsetting lecture. sample_n() has been superseded in favor of slice_sample()

[carriewright11]

The subsetting lecture is before the data classes lecture, where you wrote this comment, so it will have already been introduced.

[avahoffman]

Ok Great. This was part of a code review for @ehumph -- whoever gets to it first, let's get this fixed :)