servo: transmission: torrent-done: fix typo

hosts/by-name/servo/services/transmission/torrent-done

@@ -66,7 +66,7 @@ destructive find "$MEDIA_DIR/" -type f \(\
   -o -iname 'YTS*.com.txt' \
   \) -exec rm {} \;
 
-if ! [ -b "${TR_NO_HARDLINK}" ]; then
+if ! [ -n "${TR_NO_HARDLINK}" ]; then
   # dedupe the whole media library.
   # yeah, a bit excessive: move this to a cron job if that's problematic
   #   or make it run with only 1/N probability, etc.