Confusing error in the list concept

[mpizenberg]

Someone was confused by a typo in the list concept https://www.reddit.com/r/elm/comments/rvdz3p/help_with_syntax_of_case_and/

case list of
    x :: xs -> "Non-empty list with head: " ++ head
    -- instead of
    x :: xs -> "Non-empty list with head: " ++ x

Apparently this bug should have been fixed by #441 but this PR only fixes the concepts/lists/introduction.md file. So we need to also fix the concepts/lists/about.md file and the exercises/concept/tracks-on-tracks-on-tracks/.docs/introduction.md file.

A bit annoying to have all three files to fix but it's like that because all 3 files could be different.