Rex::Command::Run: separate POD =item(s) with newline

- The POD parser built into MetaCPAN, as well as the 'perlpod'
  command, need blank lines before and after POD commands (lines that
  start with '='), or they won't format the POD correctly; you can see
  the second '=item' POD command in the output of 'perlpod Run.pm' or
  in the current MetaCPAN page for Rex::Command::Run.  This is noted
  in the 'perlpod' POD page, in the "Hints for Writing Pod" section