[bug#70542] Missing reference in cover letter

[Richard Sent]

Oops, forgot to include the link to [1] in the cover letter.

(If you see this Felix, nothing's wrong with your code! :) I just needed
an example of how it's currently done.)

[1] https://lists.gnu.org/archive/html/guix-devel/2024-04/msg00233.html

-- 
Take it easy,
Richard Sent
Making my computer weirder one commit at a time.