[isolate] Fix flag comment to use single dash
[isolate] Fix flag comment to use single dash

Change-Id: I9962cb7faea3273c072e3647b9a37e778c5d409c
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/2103013
Commit-Queue: Ye Kuang <1cd89f22f3967fc3dcb883b53940be4b98ec483f@google.com>
Commit-Queue: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>
Auto-Submit: Ye Kuang <1cd89f22f3967fc3dcb883b53940be4b98ec483f@google.com>
Reviewed-by: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go