cros_vm_start: add an example of --vnc

Add an example of a valid argument to pass to --vnc so people don't
have to guess.

BUG=None
TEST=`./bin/cros_vm_start --help` looks good

Change-Id: I39fac310e02df798c80de060718a53037fa7784b
Reviewed-on: https://chromium-review.googlesource.com/179180
Reviewed-by: Prashanth B <beeps@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
1 file changed