Revert to double-colon namespace for `update_rank_list()`

[matt-dray]

The CRAN-released version of {sortable} does not export update_rank_list() because of malformed {roxygen2}: the @examples tag was missing.

Here I've had to call the function with sortable:::update_rank_list() (i.e. with the triple-colon operator).

Revert to the double-colon operator on the next release of {sortable}.