Show use of `which()` on NAs

[epartan]

When we teach from the materials, in lesson 6 https://hbctraining.github.io/Intro-to-R-flipped/lessons/06_data-wrangling.html we add an aside about which() using test (1 NA 2 3 NA 4) to show that it removes NAs. This would be helpful to have written into the material.