npcx_monitor: Fix readme instructions

gsutil is deprecated and has been replaced by "gcloud storage". Update
instructions and correct the access enum value.

BUG=none
TEST=View in gitiles

Change-Id: Ic60cd46145e364623f6e90fe95ce2f5c9bfebed1
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/7833000
Tested-by: Keith Short <keithshort@chromium.org>
Reviewed-by: Abe Levkoy <alevkoy@chromium.org>
Reviewed-by: Wai-Hong Tam <waihong@google.com>
Code-Coverage: Zoss <zoss-cl-coverage@prod.google.com>
Commit-Queue: Keith Short <keithshort@chromium.org>