update string->url doc with better regexp
update string->url doc with better regexp

to sync with https://github.com/racket/racket/commit/e6a0caa1471820ae1f8a76985858b197a35763a3

mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket