devserver: Minor fix to option help string.

Makes it clear that starting with --port=0 will always work.

BUG=chromium:322436
TEST=None

Change-Id: I68e2d4d34a3eb3c98e96c2898b3e0e11354f40ad
Reviewed-on: https://chromium-review.googlesource.com/186696
Tested-by: Gilad Arnold <garnold@chromium.org>
Reviewed-by: Don Garrett <dgarrett@chromium.org>
Commit-Queue: Gilad Arnold <garnold@chromium.org>
1 file changed