cros flash: fix usage message
cros flash: fix usage message

BUG=chromium:345899
TEST=Visually inspect the output of `cros flash -h`

Change-Id: I95c268c467451eb09ee435ada0922bb7dafbd713
Reviewed-on: https://chromium-review.googlesource.com/187511
Reviewed-by: mukesh agrawal <7b3e7a141c6975a7a08771090d3222103f21a0f2@chromium.org>
Commit-Queue: Yu-Ju Hong <52dc720c411f8e2156e608c1a8b894a917489667@chromium.org>
Tested-by: Yu-Ju Hong <52dc720c411f8e2156e608c1a8b894a917489667@chromium.org>

bpsinc-native/src_third_party_chromite,bpsinc-native/src_third_party_chromite,chadversary/chromiumos.chromite,chadversary/chromiumos.chromite,bpsinc-native/src_third_party_chromite