Flags can be passed as "--any_flag" or "--any-flag". It seems that
people generally prefer the second form, but our help outputs the first
one. Avoid confusion by outputting the second form.

R=bmeurer@chromium.org

Change-Id: I21e07a7d2484ae78ccd27736f8373d53eb312818
Reviewed-on: https://chromium-review.googlesource.com/850692Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50376}