crosh: add clarification to usage message of "swap" command. BUG=none TEST=none Change-Id: Ic4525191a062cb7af50710ee8288cf41c7b3a17c Reviewed-on: https://chromium-review.googlesource.com/207996 Reviewed-by: Sonny Rao <sonnyrao@chromium.org> Commit-Queue: Luigi Semenzato <semenzato@chromium.org> Tested-by: Luigi Semenzato <semenzato@chromium.org>