update-prebuilts.sh: add a script help to download the script

to avoid problem caused by typo or any other case

Test: test with the download of 6423191 prebuilts

Signed-off-by: Yongqin Liu <yongqin.liu@linaro.org>
Change-Id: I253dfd591620a57c54db551715b2ee76550bb420
1 file changed