docs: add amend step for Gerrit review updates

Add an explicit amend command in the workflow so contributors reuse
the same Change-Id and publish a new patch set on the existing review.

Change-Id: Ica155d4c52e55f04d62dd2abda74881d32927898
Signed-off-by: Hariharan Rangasamy <hariharanr@ami.com>