iteflash.md: revised the steps in 'Prerequisites for CR50 CCD'
iteflash.md: revised the steps in 'Prerequisites for CR50 CCD'

BRANCH=none
BUG=b:124388894
TEST=checked the document through Gitiles.

Change-Id: I11fe46a8900ec59edee799157358698e4c0e1cc6
Signed-off-by: Namyoon Woo <e8732cefb861f07768f47df788a8e8eb75efc09a@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/1555026
Reviewed-by: Matthew Blecker <d18f4466f2802f9c78c5c14a42b8c21fdbfabfcd@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec