Adjust cats section to reflect changes to color highlighting in RStudio

[ndporter]

When the cats dataframe is initially defined (https://swcarpentry.github.io/r-novice-gapminder/04-data-structures-part1/index.html), new versions of RStudio will now higlight "black" in black by default (color highlighting - see screenshot). While not a high priority, we should either consider changing to a color with no auto-highlighting or updating the screenshots to show the highlighting.