Created by: mtthwhggns
Following @joe-watkins spotting a typo and making a good suggestion, this makes two tiny changes;
- Fixes the typo, also replacing the word with a clearer word.
- Adds the more technically correct term "colour vision deficiencies" to the introductory section, although sticks with the more widely used "colour blindness" everywhere else.
@svinkle I'd started amending my last PR, but you just merged it!