It will work either way, but with the map functions in this case you can just put the name of the function at the end. You don't need to call it as an anonymous function. Also, you should escape the period in the file extension, otherwise it would match any character. I usually put an end of line in the regex too for good measure.
files <- purrr::map_dfr(list.files(pattern = "\\.txt$"), read_tsv)