fix typo introduced in b56542a9f

Thanks to @siteshwar for spotting.

https://github.com/fish-shell/fish-shell/commit/b56542a9f195358b1ac33716bde0d731b8e70871#commitcomment-4083982
https://github.com/fish-shell/fish-shell/commit/b56542a9f195358b1ac33716bde0d731b8e70871#commitcomment-4083982