clarify usage of -x/--extract option

This clarifies the help menu entry for -x and specifies the
actual longopt.

BUG=none
BRANCH=none
TEST=compile tested

Change-Id: I875e1d4cef989186e1e5f09db555362c7c554654
Reviewed-on: https://chromium-review.googlesource.com/172484
Reviewed-by: Yusuf Mohsinally <mohsinally@chromium.org>
Commit-Queue: David Hendricks <dhendrix@chromium.org>
Tested-by: David Hendricks <dhendrix@chromium.org>
1 file changed