refresh: Fix format string that would lead '-l' to print incorrect numbers.

refresh: Fix format string that would lead '-l' to print incorrect numbers.

The skip "~*" argument was misplaced, leading the number of dependents
to be skipped (instead of the number of covering packages.)  Thus, we'd
get:

  $ guix refresh -l ocaml4.02-ppx-deriving@4.1
  Building the following package would ensure 1 dependent packages are rebuilt: bap@1.3.0

instead of:

  Building the following package would ensure 26 dependent packages are rebuilt: bap@1.3.0

* guix/scripts/refresh.scm (list-dependents): Move "~*" in the right
place, to skip (length covering) rather than (length dependents).