docs: add reference for lesmis (#43)

The DOI at https://dl.acm.org/doi/book/10.1145/164984 does not work

Author: maelle

R/files.R

@@ -5,6 +5,8 @@
 #'
 #' @return A string indicating an absolute path to a file.
 #' @name lesmis
+#' @references D. E. Knuth, The Stanford GraphBase: A Platform for Combinatorial Computing, Addison-Wesley, Reading, MA (1993).
+#' \url{https://www-cs-faculty.stanford.edu/~knuth/sgb.html}
 #' @export
 lesmis_gml <- function() {
   system.file("files/lesmis.gml", package = "igraphdata", mustWork = TRUE)