2api: Correct the comment of vb2api_disable_developer_mode()

It should be VB2_NV_DISABLE_DEV_REQUEST, not VB2_NV_DIAG_REQUEST.

BUG=b:172339016
TEST=none
BRANCH=none

Change-Id: Id35ea2680dd55d830991cf118f4e9c97f52eb0cb
Signed-off-by: Hsuan Ting Chen <roccochen@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/3532539
Reviewed-by: Yu-Ping Wu <yupingso@chromium.org>