Fix arguments description for `when` parameter

`last_day` to `last-day` and so on. One fix at a time.

R/cranlogs.R

@@ -17,7 +17,7 @@ top_url   <- paste0(base_url, "top/")
 #'   or \code{NULL} for a sum of downloads for all packages.
 #'   Alternatively, it can also be \code{"R"}, to query downloads
 #'   of R itself. \code{"R"} cannot be mixed with packages.
-#' @param when \code{last_day}, \code{last_week} or \code{last_month}.
+#' @param when \code{last-day}, \code{last-week} or \code{last-month}.
 #'   If this is given, then \code{from} and \code{to} are ignored.
 #' @param from Start date, in \code{yyyy-mm-dd} format, or
 #'   \code{last-day}. It is ignored if \code{when} is given.