doc: replace --help with -h

The scripts display help information by -h parameter, so
need to replace --help in the documentation with -h.

Signed-off-by: Yefeng Lin <yefeng.lin@arm.com>
Reviewed-by: Liangxing Wang <liangxing.wang@arm.com>

Change-Id: Iaa8babe7e74d423b768adb79ec9fe22b7096422a