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>
1 file changed