Documentation update

[Wiktor Włodzimierz Gonczaronek]

Hi.
I've come across your project 
(https://tiswww.cwru.edu/php/chet/readline/rltop.html) and I've noticed 
that the link to homebrew formula is broken. It seems to be grouped by 
the first letter, so the link should be: 
https://github.com/Homebrew/homebrew-core/blob/master/Formula/r/readline.rb

Thanks for the project,

kind regards!